Solve 3x-(k+1)y=20text{and}(k+2)x-10y=40

$$3 x - ( k + 1 ) y = 20 \text { and } ( k + 2 ) x - 10 y = 40$$

Solve for x, y (complex solution)

$$\left\{\begin{matrix}x=\frac{40}{k+7}\text{, }y=-\frac{20}{k+7}\text{, }&k\neq -7\\x=\frac{5\left(y+4\right)}{3}\text{, }y\in \mathrm{C}\text{, }&k=4\end{matrix}\right.$$

Steps Using Substitution

Consider the first equation. Use the distributive property to multiply k+1 by y.

$$3x-\left(ky+y\right)=20$$

To find the opposite of ky+y, find the opposite of each term.

$$3x-ky-y=20$$

Combine all terms containing x,y.

$$3x+\left(-k-1\right)y=20$$

Consider the second equation. Use the distributive property to multiply k+2 by x.

$$kx+2x-10y=40$$

Combine all terms containing x,y.

$$\left(k+2\right)x-10y=40$$

To solve a pair of equations using substitution, first solve one of the equations for one of the variables. Then substitute the result for that variable in the other equation.

$$3x+\left(-k-1\right)y=20,\left(k+2\right)x-10y=40$$

Choose one of the equations and solve it for x by isolating x on the left hand side of the equal sign.

$$3x+\left(-k-1\right)y=20$$

Add \left(k+1\right)y to both sides of the equation.

$$3x=\left(k+1\right)y+20$$

Divide both sides by 3.

$$x=\frac{1}{3}\left(\left(k+1\right)y+20\right)$$

Multiply \frac{1}{3} times yk+y+20.

$$x=\frac{k+1}{3}y+\frac{20}{3}$$

Substitute \frac{yk+y+20}{3} for x in the other equation, \left(k+2\right)x-10y=40.

$$\left(k+2\right)\left(\frac{k+1}{3}y+\frac{20}{3}\right)-10y=40$$

Multiply k+2 times \frac{yk+y+20}{3}.

$$\frac{\left(k+1\right)\left(k+2\right)}{3}y+\frac{20k+40}{3}-10y=40$$

Add \frac{\left(k+2\right)\left(k+1\right)y}{3} to -10y.

$$\frac{\left(k-4\right)\left(k+7\right)}{3}y+\frac{20k+40}{3}=40$$

Subtract \frac{40+20k}{3} from both sides of the equation.

$$\frac{\left(k-4\right)\left(k+7\right)}{3}y=\frac{80-20k}{3}$$

Divide both sides by \frac{\left(-4+k\right)\left(7+k\right)}{3}.

$$y=-\frac{20}{k+7}$$

Substitute -\frac{20}{7+k} for y in x=\frac{k+1}{3}y+\frac{20}{3}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=\frac{k+1}{3}\left(-\frac{20}{k+7}\right)+\frac{20}{3}$$

Multiply \frac{k+1}{3} times -\frac{20}{7+k}.

$$x=-\frac{20\left(k+1\right)}{3\left(k+7\right)}+\frac{20}{3}$$

Add \frac{20}{3} to -\frac{20\left(k+1\right)}{3\left(7+k\right)}.

$$x=\frac{40}{k+7}$$

The system is now solved.

$$x=\frac{40}{k+7},y=-\frac{20}{k+7}$$

Steps Using Matrices

Consider the first equation. Use the distributive property to multiply k+1 by y.

$$3x-\left(ky+y\right)=20$$

To find the opposite of ky+y, find the opposite of each term.

$$3x-ky-y=20$$

Combine all terms containing x,y.

$$3x+\left(-k-1\right)y=20$$

Consider the second equation. Use the distributive property to multiply k+2 by x.

$$kx+2x-10y=40$$

Combine all terms containing x,y.

$$\left(k+2\right)x-10y=40$$

Put the equations in standard form and then use matrices to solve the system of equations.

$$3x+\left(-k-1\right)y=20,\left(k+2\right)x-10y=40$$

Write the equations in matrix form.

$$\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}20\\40\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right).

$$inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}20\\40\end{matrix}\right)$$

The product of a matrix and its inverse is the identity matrix.

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}20\\40\end{matrix}\right)$$

Multiply the matrices on the left hand side of the equal sign.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}20\\40\end{matrix}\right)$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the inverse matrix is \left(\begin{matrix}\frac{d}{ad-bc}&\frac{-b}{ad-bc}\\\frac{-c}{ad-bc}&\frac{a}{ad-bc}\end{matrix}\right), so the matrix equation can be rewritten as a matrix multiplication problem.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{10}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}&-\frac{-k-1}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}\\-\frac{k+2}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}&\frac{3}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}\end{matrix}\right)\left(\begin{matrix}20\\40\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{10}{\left(k-4\right)\left(k+7\right)}&\frac{k+1}{\left(k-4\right)\left(k+7\right)}\\-\frac{k+2}{\left(k-4\right)\left(k+7\right)}&\frac{3}{\left(k-4\right)\left(k+7\right)}\end{matrix}\right)\left(\begin{matrix}20\\40\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\left(-\frac{10}{\left(k-4\right)\left(k+7\right)}\right)\times 20+\frac{k+1}{\left(k-4\right)\left(k+7\right)}\times 40\\\left(-\frac{k+2}{\left(k-4\right)\left(k+7\right)}\right)\times 20+\frac{3}{\left(k-4\right)\left(k+7\right)}\times 40\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{40}{k+7}\\-\frac{20}{k+7}\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=\frac{40}{k+7},y=-\frac{20}{k+7}$$

Steps Using Elimination

Consider the first equation. Use the distributive property to multiply k+1 by y.

$$3x-\left(ky+y\right)=20$$

To find the opposite of ky+y, find the opposite of each term.

$$3x-ky-y=20$$

Combine all terms containing x,y.

$$3x+\left(-k-1\right)y=20$$

Consider the second equation. Use the distributive property to multiply k+2 by x.

$$kx+2x-10y=40$$

Combine all terms containing x,y.

$$\left(k+2\right)x-10y=40$$

In order to solve by elimination, coefficients of one of the variables must be the same in both equations so that the variable will cancel out when one equation is subtracted from the other.

$$3x+\left(-k-1\right)y=20,\left(k+2\right)x-10y=40$$

To make 3x and \left(k+2\right)x equal, multiply all terms on each side of the first equation by k+2 and all terms on each side of the second by 3.

$$\left(k+2\right)\times 3x+\left(k+2\right)\left(-k-1\right)y=\left(k+2\right)\times 20,3\left(k+2\right)x+3\left(-10\right)y=3\times 40$$

Simplify.

$$\left(3k+6\right)x+\left(-\left(k+1\right)\left(k+2\right)\right)y=20k+40,\left(3k+6\right)x-30y=120$$

Subtract \left(3k+6\right)x-30y=120 from \left(3k+6\right)x+\left(-\left(k+1\right)\left(k+2\right)\right)y=20k+40 by subtracting like terms on each side of the equal sign.

$$\left(3k+6\right)x+\left(-3k-6\right)x+\left(-\left(k+1\right)\left(k+2\right)\right)y+30y=20k+40-120$$

Add 3\left(2+k\right)x to -6x-3xk. Terms 3\left(2+k\right)x and -6x-3xk cancel out, leaving an equation with only one variable that can be solved.

$$\left(-\left(k+1\right)\left(k+2\right)\right)y+30y=20k+40-120$$

Add -\left(k+2\right)\left(k+1\right)y to 30y.

$$\left(4-k\right)\left(k+7\right)y=20k+40-120$$

Add 20k+40 to -120.

$$\left(4-k\right)\left(k+7\right)y=20k-80$$

Divide both sides by \left(4-k\right)\left(7+k\right).

$$y=-\frac{20}{k+7}$$

Substitute -\frac{20}{7+k} for y in \left(k+2\right)x-10y=40. Because the resulting equation contains only one variable, you can solve for x directly.

$$\left(k+2\right)x-10\left(-\frac{20}{k+7}\right)=40$$

Multiply -10 times -\frac{20}{7+k}.

$$\left(k+2\right)x+\frac{200}{k+7}=40$$

Subtract \frac{200}{7+k} from both sides of the equation.

$$\left(k+2\right)x=\frac{40\left(k+2\right)}{k+7}$$

Divide both sides by k+2.

$$x=\frac{40}{k+7}$$

The system is now solved.

$$x=\frac{40}{k+7},y=-\frac{20}{k+7}$$

Solve for x, y

$$\left\{\begin{matrix}x=\frac{40}{k+7}\text{, }y=-\frac{20}{k+7}\text{, }&k\neq -7\text{ and }k\neq 4\\x=\frac{5\left(y+4\right)}{3}\text{, }y\in \mathrm{R}\text{, }&k=4\end{matrix}\right.$$

Steps Using Substitution

Consider the first equation. Use the distributive property to multiply k+1 by y.

$$3x-\left(ky+y\right)=20$$

To find the opposite of ky+y, find the opposite of each term.

$$3x-ky-y=20$$

Combine all terms containing x,y.

$$3x+\left(-k-1\right)y=20$$

Consider the second equation. Use the distributive property to multiply k+2 by x.

$$kx+2x-10y=40$$

Combine all terms containing x,y.

$$\left(k+2\right)x-10y=40$$

To solve a pair of equations using substitution, first solve one of the equations for one of the variables. Then substitute the result for that variable in the other equation.

$$3x+\left(-k-1\right)y=20,\left(k+2\right)x-10y=40$$

Choose one of the equations and solve it for x by isolating x on the left hand side of the equal sign.

$$3x+\left(-k-1\right)y=20$$

Add \left(k+1\right)y to both sides of the equation.

$$3x=\left(k+1\right)y+20$$

Divide both sides by 3.

$$x=\frac{1}{3}\left(\left(k+1\right)y+20\right)$$

Multiply \frac{1}{3} times yk+y+20.

$$x=\frac{k+1}{3}y+\frac{20}{3}$$

Substitute \frac{yk+y+20}{3} for x in the other equation, \left(k+2\right)x-10y=40.

$$\left(k+2\right)\left(\frac{k+1}{3}y+\frac{20}{3}\right)-10y=40$$

Multiply k+2 times \frac{yk+y+20}{3}.

$$\frac{\left(k+1\right)\left(k+2\right)}{3}y+\frac{20k+40}{3}-10y=40$$

Add \frac{\left(k+2\right)\left(k+1\right)y}{3} to -10y.

$$\frac{\left(k-4\right)\left(k+7\right)}{3}y+\frac{20k+40}{3}=40$$

Subtract \frac{40+20k}{3} from both sides of the equation.

$$\frac{\left(k-4\right)\left(k+7\right)}{3}y=\frac{80-20k}{3}$$

Divide both sides by \frac{\left(-4+k\right)\left(7+k\right)}{3}.

$$y=-\frac{20}{k+7}$$

Substitute -\frac{20}{7+k} for y in x=\frac{k+1}{3}y+\frac{20}{3}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=\frac{k+1}{3}\left(-\frac{20}{k+7}\right)+\frac{20}{3}$$

Multiply \frac{k+1}{3} times -\frac{20}{7+k}.

$$x=-\frac{20\left(k+1\right)}{3\left(k+7\right)}+\frac{20}{3}$$

Add \frac{20}{3} to -\frac{20\left(k+1\right)}{3\left(7+k\right)}.

$$x=\frac{40}{k+7}$$

The system is now solved.

$$x=\frac{40}{k+7},y=-\frac{20}{k+7}$$

Steps Using Matrices

Consider the first equation. Use the distributive property to multiply k+1 by y.

$$3x-\left(ky+y\right)=20$$

To find the opposite of ky+y, find the opposite of each term.

$$3x-ky-y=20$$

Combine all terms containing x,y.

$$3x+\left(-k-1\right)y=20$$

Consider the second equation. Use the distributive property to multiply k+2 by x.

$$kx+2x-10y=40$$

Combine all terms containing x,y.

$$\left(k+2\right)x-10y=40$$

Put the equations in standard form and then use matrices to solve the system of equations.

$$3x+\left(-k-1\right)y=20,\left(k+2\right)x-10y=40$$

Write the equations in matrix form.

$$\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}20\\40\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right).

$$inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}20\\40\end{matrix}\right)$$

The product of a matrix and its inverse is the identity matrix.

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}20\\40\end{matrix}\right)$$

Multiply the matrices on the left hand side of the equal sign.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}3&-k-1\\k+2&-10\end{matrix}\right))\left(\begin{matrix}20\\40\end{matrix}\right)$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the inverse matrix is \left(\begin{matrix}\frac{d}{ad-bc}&\frac{-b}{ad-bc}\\\frac{-c}{ad-bc}&\frac{a}{ad-bc}\end{matrix}\right), so the matrix equation can be rewritten as a matrix multiplication problem.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{10}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}&-\frac{-k-1}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}\\-\frac{k+2}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}&\frac{3}{3\left(-10\right)-\left(-k-1\right)\left(k+2\right)}\end{matrix}\right)\left(\begin{matrix}20\\40\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{10}{\left(k-4\right)\left(k+7\right)}&\frac{k+1}{\left(k-4\right)\left(k+7\right)}\\-\frac{k+2}{\left(k-4\right)\left(k+7\right)}&\frac{3}{\left(k-4\right)\left(k+7\right)}\end{matrix}\right)\left(\begin{matrix}20\\40\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\left(-\frac{10}{\left(k-4\right)\left(k+7\right)}\right)\times 20+\frac{k+1}{\left(k-4\right)\left(k+7\right)}\times 40\\\left(-\frac{k+2}{\left(k-4\right)\left(k+7\right)}\right)\times 20+\frac{3}{\left(k-4\right)\left(k+7\right)}\times 40\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{40}{k+7}\\-\frac{20}{k+7}\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=\frac{40}{k+7},y=-\frac{20}{k+7}$$

Steps Using Elimination

Consider the first equation. Use the distributive property to multiply k+1 by y.

$$3x-\left(ky+y\right)=20$$

To find the opposite of ky+y, find the opposite of each term.

$$3x-ky-y=20$$

Combine all terms containing x,y.

$$3x+\left(-k-1\right)y=20$$

Consider the second equation. Use the distributive property to multiply k+2 by x.

$$kx+2x-10y=40$$

Combine all terms containing x,y.

$$\left(k+2\right)x-10y=40$$

In order to solve by elimination, coefficients of one of the variables must be the same in both equations so that the variable will cancel out when one equation is subtracted from the other.

$$3x+\left(-k-1\right)y=20,\left(k+2\right)x-10y=40$$

To make 3x and \left(k+2\right)x equal, multiply all terms on each side of the first equation by k+2 and all terms on each side of the second by 3.

$$\left(k+2\right)\times 3x+\left(k+2\right)\left(-k-1\right)y=\left(k+2\right)\times 20,3\left(k+2\right)x+3\left(-10\right)y=3\times 40$$

Simplify.

$$\left(3k+6\right)x+\left(-\left(k+1\right)\left(k+2\right)\right)y=20k+40,\left(3k+6\right)x-30y=120$$

Subtract \left(3k+6\right)x-30y=120 from \left(3k+6\right)x+\left(-\left(k+1\right)\left(k+2\right)\right)y=20k+40 by subtracting like terms on each side of the equal sign.

$$\left(3k+6\right)x+\left(-3k-6\right)x+\left(-\left(k+1\right)\left(k+2\right)\right)y+30y=20k+40-120$$

Add 3\left(2+k\right)x to -6x-3xk. Terms 3\left(2+k\right)x and -6x-3xk cancel out, leaving an equation with only one variable that can be solved.

$$\left(-\left(k+1\right)\left(k+2\right)\right)y+30y=20k+40-120$$

Add -\left(k+2\right)\left(k+1\right)y to 30y.

$$\left(4-k\right)\left(k+7\right)y=20k+40-120$$

Add 20k+40 to -120.

$$\left(4-k\right)\left(k+7\right)y=20k-80$$

Divide both sides by \left(4-k\right)\left(7+k\right).

$$y=-\frac{20}{k+7}$$

Substitute -\frac{20}{7+k} for y in \left(k+2\right)x-10y=40. Because the resulting equation contains only one variable, you can solve for x directly.

$$\left(k+2\right)x-10\left(-\frac{20}{k+7}\right)=40$$

Multiply -10 times -\frac{20}{7+k}.

$$\left(k+2\right)x+\frac{200}{k+7}=40$$

Subtract \frac{200}{7+k} from both sides of the equation.

$$\left(k+2\right)x=\frac{40\left(k+2\right)}{k+7}$$

Divide both sides by k+2.

$$x=\frac{40}{k+7}$$

The system is now solved.

$$x=\frac{40}{k+7},y=-\frac{20}{k+7}$$