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7x+2y=192,8x+y=204
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.
7x+2y=192
Choose one of the equations and solve it for x by isolating x on the left hand side of the equal sign.
7x=-2y+192
Subtract 2y from both sides of the equation.
x=\frac{1}{7}\left(-2y+192\right)
Divide both sides by 7.
x=-\frac{2}{7}y+\frac{192}{7}
Multiply \frac{1}{7} times -2y+192.
8\left(-\frac{2}{7}y+\frac{192}{7}\right)+y=204
Substitute \frac{-2y+192}{7} for x in the other equation, 8x+y=204.
-\frac{16}{7}y+\frac{1536}{7}+y=204
Multiply 8 times \frac{-2y+192}{7}.
-\frac{9}{7}y+\frac{1536}{7}=204
Add -\frac{16y}{7} to y.
-\frac{9}{7}y=-\frac{108}{7}
Subtract \frac{1536}{7} from both sides of the equation.
y=12
Divide both sides of the equation by -\frac{9}{7}, which is the same as multiplying both sides by the reciprocal of the fraction.
x=-\frac{2}{7}\times 12+\frac{192}{7}
Substitute 12 for y in x=-\frac{2}{7}y+\frac{192}{7}. Because the resulting equation contains only one variable, you can solve for x directly.
x=\frac{-24+192}{7}
Multiply -\frac{2}{7} times 12.
x=24
Add \frac{192}{7} to -\frac{24}{7} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.
x=24,y=12
The system is now solved.
7x+2y=192,8x+y=204
Put the equations in standard form and then use matrices to solve the system of equations.
\left(\begin{matrix}7&2\\8&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}192\\204\end{matrix}\right)
Write the equations in matrix form.
inverse(\left(\begin{matrix}7&2\\8&1\end{matrix}\right))\left(\begin{matrix}7&2\\8&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}7&2\\8&1\end{matrix}\right))\left(\begin{matrix}192\\204\end{matrix}\right)
Left multiply the equation by the inverse matrix of \left(\begin{matrix}7&2\\8&1\end{matrix}\right).
\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}7&2\\8&1\end{matrix}\right))\left(\begin{matrix}192\\204\end{matrix}\right)
The product of a matrix and its inverse is the identity matrix.
\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}7&2\\8&1\end{matrix}\right))\left(\begin{matrix}192\\204\end{matrix}\right)
Multiply the matrices on the left hand side of the equal sign.
\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{7-2\times 8}&-\frac{2}{7-2\times 8}\\-\frac{8}{7-2\times 8}&\frac{7}{7-2\times 8}\end{matrix}\right)\left(\begin{matrix}192\\204\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{1}{9}&\frac{2}{9}\\\frac{8}{9}&-\frac{7}{9}\end{matrix}\right)\left(\begin{matrix}192\\204\end{matrix}\right)
Do the arithmetic.
\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{1}{9}\times 192+\frac{2}{9}\times 204\\\frac{8}{9}\times 192-\frac{7}{9}\times 204\end{matrix}\right)
Multiply the matrices.
\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}24\\12\end{matrix}\right)
Do the arithmetic.
x=24,y=12
Extract the matrix elements x and y.
7x+2y=192,8x+y=204
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.
8\times 7x+8\times 2y=8\times 192,7\times 8x+7y=7\times 204
To make 7x 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 7.
56x+16y=1536,56x+7y=1428
Simplify.
56x-56x+16y-7y=1536-1428
Subtract 56x+7y=1428 from 56x+16y=1536 by subtracting like terms on each side of the equal sign.
16y-7y=1536-1428
Add 56x to -56x. Terms 56x and -56x cancel out, leaving an equation with only one variable that can be solved.
9y=1536-1428
Add 16y to -7y.
9y=108
Add 1536 to -1428.
y=12
Divide both sides by 9.
8x+12=204
Substitute 12 for y in 8x+y=204. Because the resulting equation contains only one variable, you can solve for x directly.
8x=192
Subtract 12 from both sides of the equation.
x=24
Divide both sides by 8.
x=24,y=12
The system is now solved.