Solve {l}{21x+7y=42}{-5x+5y=10}

$$\left\{ \begin{array} { l } { 21 x + 7 y = 42 } \\ { - 5 x + 5 y = 10 } \end{array} \right.$$

Solve for x, y

$$x=1$$

$$y=3$$

Steps Using Substitution

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.

$$21x+7y=42,-5x+5y=10$$

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

$$21x+7y=42$$

Subtract 7y from both sides of the equation.

$$21x=-7y+42$$

Divide both sides by 21.

$$x=\frac{1}{21}\left(-7y+42\right)$$

Multiply \frac{1}{21} times -7y+42.

$$x=-\frac{1}{3}y+2$$

Substitute -\frac{y}{3}+2 for x in the other equation, -5x+5y=10.

$$-5\left(-\frac{1}{3}y+2\right)+5y=10$$

Multiply -5 times -\frac{y}{3}+2.

$$\frac{5}{3}y-10+5y=10$$

Add \frac{5y}{3} to 5y.

$$\frac{20}{3}y-10=10$$

Add 10 to both sides of the equation.

$$\frac{20}{3}y=20$$

Divide both sides of the equation by \frac{20}{3}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=3$$

Substitute 3 for y in x=-\frac{1}{3}y+2. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{1}{3}\times 3+2$$

Multiply -\frac{1}{3} times 3.

$$x=-1+2$$

Add 2 to -1.

$$x=1$$

The system is now solved.

$$x=1,y=3$$

Steps Using Matrices

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

$$21x+7y=42,-5x+5y=10$$

Write the equations in matrix form.

$$\left(\begin{matrix}21&7\\-5&5\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}42\\10\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}21&7\\-5&5\end{matrix}\right).

$$inverse(\left(\begin{matrix}21&7\\-5&5\end{matrix}\right))\left(\begin{matrix}21&7\\-5&5\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}21&7\\-5&5\end{matrix}\right))\left(\begin{matrix}42\\10\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}21&7\\-5&5\end{matrix}\right))\left(\begin{matrix}42\\10\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}21&7\\-5&5\end{matrix}\right))\left(\begin{matrix}42\\10\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{5}{21\times 5-7\left(-5\right)}&-\frac{7}{21\times 5-7\left(-5\right)}\\-\frac{-5}{21\times 5-7\left(-5\right)}&\frac{21}{21\times 5-7\left(-5\right)}\end{matrix}\right)\left(\begin{matrix}42\\10\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{28}&-\frac{1}{20}\\\frac{1}{28}&\frac{3}{20}\end{matrix}\right)\left(\begin{matrix}42\\10\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{28}\times 42-\frac{1}{20}\times 10\\\frac{1}{28}\times 42+\frac{3}{20}\times 10\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}1\\3\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=1,y=3$$

Steps Using Elimination

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.

$$21x+7y=42,-5x+5y=10$$

To make 21x and -5x equal, multiply all terms on each side of the first equation by -5 and all terms on each side of the second by 21.

$$-5\times 21x-5\times 7y=-5\times 42,21\left(-5\right)x+21\times 5y=21\times 10$$

Simplify.

$$-105x-35y=-210,-105x+105y=210$$

Subtract -105x+105y=210 from -105x-35y=-210 by subtracting like terms on each side of the equal sign.

$$-105x+105x-35y-105y=-210-210$$

Add -105x to 105x. Terms -105x and 105x cancel out, leaving an equation with only one variable that can be solved.

$$-35y-105y=-210-210$$

Add -35y to -105y.

$$-140y=-210-210$$

Add -210 to -210.

$$-140y=-420$$

Divide both sides by -140.

$$y=3$$

Substitute 3 for y in -5x+5y=10. Because the resulting equation contains only one variable, you can solve for x directly.

$$-5x+5\times 3=10$$

Multiply 5 times 3.

$$-5x+15=10$$

Subtract 15 from both sides of the equation.

$$-5x=-5$$

Divide both sides by -5.

$$x=1$$

The system is now solved.

$$x=1,y=3$$