Solve {l}{-8x+4y=24}{-7x+7y=28}

$$\left\{ \begin{array} { l } { - 8 x + 4 y = 24 } \\ { - 7 x + 7 y = 28 } \end{array} \right.$$

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.

$$-8x+4y=24,-7x+7y=28$$

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

$$-8x+4y=24$$

Subtract 4y from both sides of the equation.

$$-8x=-4y+24$$

Divide both sides by -8.

$$x=-\frac{1}{8}\left(-4y+24\right)$$

Multiply -\frac{1}{8} times -4y+24.

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

Substitute \frac{y}{2}-3 for x in the other equation, -7x+7y=28.

$$-7\left(\frac{1}{2}y-3\right)+7y=28$$

Multiply -7 times \frac{y}{2}-3.

$$-\frac{7}{2}y+21+7y=28$$

Add -\frac{7y}{2} to 7y.

$$\frac{7}{2}y+21=28$$

Subtract 21 from both sides of the equation.

$$\frac{7}{2}y=7$$

Divide both sides of the equation by \frac{7}{2}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=2$$

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

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

Multiply \frac{1}{2} times 2.

$$x=1-3$$

Add -3 to 1.

$$x=-2$$

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.

$$-8x+4y=24,-7x+7y=28$$

Write the equations in matrix form.

$$\left(\begin{matrix}-8&4\\-7&7\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}24\\28\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}-8&4\\-7&7\end{matrix}\right).

$$inverse(\left(\begin{matrix}-8&4\\-7&7\end{matrix}\right))\left(\begin{matrix}-8&4\\-7&7\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-8&4\\-7&7\end{matrix}\right))\left(\begin{matrix}24\\28\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-8&4\\-7&7\end{matrix}\right))\left(\begin{matrix}24\\28\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-8&4\\-7&7\end{matrix}\right))\left(\begin{matrix}24\\28\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{7}{-8\times 7-4\left(-7\right)}&-\frac{4}{-8\times 7-4\left(-7\right)}\\-\frac{-7}{-8\times 7-4\left(-7\right)}&-\frac{8}{-8\times 7-4\left(-7\right)}\end{matrix}\right)\left(\begin{matrix}24\\28\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{4}&\frac{1}{7}\\-\frac{1}{4}&\frac{2}{7}\end{matrix}\right)\left(\begin{matrix}24\\28\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{4}\times 24+\frac{1}{7}\times 28\\-\frac{1}{4}\times 24+\frac{2}{7}\times 28\end{matrix}\right)$$

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.

$$-8x+4y=24,-7x+7y=28$$

To make -8x and -7x equal, multiply all terms on each side of the first equation by -7 and all terms on each side of the second by -8.

$$-7\left(-8\right)x-7\times 4y=-7\times 24,-8\left(-7\right)x-8\times 7y=-8\times 28$$

Simplify.

$$56x-28y=-168,56x-56y=-224$$

Subtract 56x-56y=-224 from 56x-28y=-168 by subtracting like terms on each side of the equal sign.

$$56x-56x-28y+56y=-168+224$$

Add 56x to -56x. Terms 56x and -56x cancel out, leaving an equation with only one variable that can be solved.

$$-28y+56y=-168+224$$

Add -28y to 56y.

$$28y=-168+224$$

Add -168 to 224.

$$28y=56$$

Divide both sides by 28.

$$y=2$$

Substitute 2 for y in -7x+7y=28. Because the resulting equation contains only one variable, you can solve for x directly.

$$-7x+7\times 2=28$$

Multiply 7 times 2.

$$-7x+14=28$$

Subtract 14 from both sides of the equation.

$$-7x=14$$

Divide both sides by -7.

$$x=-2$$

The system is now solved.

$$x=-2,y=2$$