Solve {l}{-2x-3y=-10}{-8x+10y=4}

$$\left. \begin{array} { l } { - 2 x - 3 y = - 10 } \\ { - 8 x + 10 y = 4 } \end{array} \right.$$

Solve for x, y

$$x=2$$

$$y=2$$

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.

$$-2x-3y=-10,-8x+10y=4$$

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

$$-2x-3y=-10$$

Add 3y to both sides of the equation.

$$-2x=3y-10$$

Divide both sides by -2.

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

Multiply -\frac{1}{2} times 3y-10.

$$x=-\frac{3}{2}y+5$$

Substitute -\frac{3y}{2}+5 for x in the other equation, -8x+10y=4.

$$-8\left(-\frac{3}{2}y+5\right)+10y=4$$

Multiply -8 times -\frac{3y}{2}+5.

$$12y-40+10y=4$$

Add 12y to 10y.

$$22y-40=4$$

Add 40 to both sides of the equation.

$$22y=44$$

Divide both sides by 22.

$$y=2$$

Substitute 2 for y in x=-\frac{3}{2}y+5. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{3}{2}\times 2+5$$

Multiply -\frac{3}{2} times 2.

$$x=-3+5$$

Add 5 to -3.

$$x=2$$

The system is now solved.

$$x=2,y=2$$

Steps Using Matrices

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

$$-2x-3y=-10,-8x+10y=4$$

Write the equations in matrix form.

$$\left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-10\\4\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right).

$$inverse(\left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right))\left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right))\left(\begin{matrix}-10\\4\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right))\left(\begin{matrix}-10\\4\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-2&-3\\-8&10\end{matrix}\right))\left(\begin{matrix}-10\\4\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{10}{-2\times 10-\left(-3\left(-8\right)\right)}&-\frac{-3}{-2\times 10-\left(-3\left(-8\right)\right)}\\-\frac{-8}{-2\times 10-\left(-3\left(-8\right)\right)}&-\frac{2}{-2\times 10-\left(-3\left(-8\right)\right)}\end{matrix}\right)\left(\begin{matrix}-10\\4\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{5}{22}&-\frac{3}{44}\\-\frac{2}{11}&\frac{1}{22}\end{matrix}\right)\left(\begin{matrix}-10\\4\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{5}{22}\left(-10\right)-\frac{3}{44}\times 4\\-\frac{2}{11}\left(-10\right)+\frac{1}{22}\times 4\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}2\\2\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=2,y=2$$

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.

$$-2x-3y=-10,-8x+10y=4$$

To make -2x and -8x equal, multiply all terms on each side of the first equation by -8 and all terms on each side of the second by -2.

$$-8\left(-2\right)x-8\left(-3\right)y=-8\left(-10\right),-2\left(-8\right)x-2\times 10y=-2\times 4$$

Simplify.

$$16x+24y=80,16x-20y=-8$$

Subtract 16x-20y=-8 from 16x+24y=80 by subtracting like terms on each side of the equal sign.

$$16x-16x+24y+20y=80+8$$

Add 16x to -16x. Terms 16x and -16x cancel out, leaving an equation with only one variable that can be solved.

$$24y+20y=80+8$$

Add 24y to 20y.

$$44y=80+8$$

Add 80 to 8.

$$44y=88$$

Divide both sides by 44.

$$y=2$$

Substitute 2 for y in -8x+10y=4. Because the resulting equation contains only one variable, you can solve for x directly.

$$-8x+10\times 2=4$$

Multiply 10 times 2.

$$-8x+20=4$$

Subtract 20 from both sides of the equation.

$$-8x=-16$$

Divide both sides by -8.

$$x=2$$

The system is now solved.

$$x=2,y=2$$