Solve {l}{4x+y=15}{5y+19x=30}

$$\left. \begin{array} { l } { 4 x + y = 15 } \\ { 5 y + 19 x = 30 } \end{array} \right.$$

Solve for x, y

$$x=45$$

$$y=-165$$

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.

$$4x+y=15,19x+5y=30$$

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

$$4x+y=15$$

Subtract y from both sides of the equation.

$$4x=-y+15$$

Divide both sides by 4.

$$x=\frac{1}{4}\left(-y+15\right)$$

Multiply \frac{1}{4} times -y+15.

$$x=-\frac{1}{4}y+\frac{15}{4}$$

Substitute \frac{-y+15}{4} for x in the other equation, 19x+5y=30.

$$19\left(-\frac{1}{4}y+\frac{15}{4}\right)+5y=30$$

Multiply 19 times \frac{-y+15}{4}.

$$-\frac{19}{4}y+\frac{285}{4}+5y=30$$

Add -\frac{19y}{4} to 5y.

$$\frac{1}{4}y+\frac{285}{4}=30$$

Subtract \frac{285}{4} from both sides of the equation.

$$\frac{1}{4}y=-\frac{165}{4}$$

Multiply both sides by 4.

$$y=-165$$

Substitute -165 for y in x=-\frac{1}{4}y+\frac{15}{4}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{1}{4}\left(-165\right)+\frac{15}{4}$$

Multiply -\frac{1}{4} times -165.

$$x=\frac{165+15}{4}$$

Add \frac{15}{4} to \frac{165}{4} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=45$$

The system is now solved.

$$x=45,y=-165$$

Steps Using Matrices

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

$$4x+y=15,19x+5y=30$$

Write the equations in matrix form.

$$\left(\begin{matrix}4&1\\19&5\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}15\\30\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}4&1\\19&5\end{matrix}\right).

$$inverse(\left(\begin{matrix}4&1\\19&5\end{matrix}\right))\left(\begin{matrix}4&1\\19&5\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}4&1\\19&5\end{matrix}\right))\left(\begin{matrix}15\\30\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}4&1\\19&5\end{matrix}\right))\left(\begin{matrix}15\\30\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}4&1\\19&5\end{matrix}\right))\left(\begin{matrix}15\\30\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{5}{4\times 5-19}&-\frac{1}{4\times 5-19}\\-\frac{19}{4\times 5-19}&\frac{4}{4\times 5-19}\end{matrix}\right)\left(\begin{matrix}15\\30\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}5&-1\\-19&4\end{matrix}\right)\left(\begin{matrix}15\\30\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}5\times 15-30\\-19\times 15+4\times 30\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}45\\-165\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=45,y=-165$$

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.

$$4x+y=15,19x+5y=30$$

To make 4x and 19x equal, multiply all terms on each side of the first equation by 19 and all terms on each side of the second by 4.

$$19\times 4x+19y=19\times 15,4\times 19x+4\times 5y=4\times 30$$

Simplify.

$$76x+19y=285,76x+20y=120$$

Subtract 76x+20y=120 from 76x+19y=285 by subtracting like terms on each side of the equal sign.

$$76x-76x+19y-20y=285-120$$

Add 76x to -76x. Terms 76x and -76x cancel out, leaving an equation with only one variable that can be solved.

$$19y-20y=285-120$$

Add 19y to -20y.

$$-y=285-120$$

Add 285 to -120.

$$-y=165$$

Divide both sides by -1.

$$y=-165$$

Substitute -165 for y in 19x+5y=30. Because the resulting equation contains only one variable, you can solve for x directly.

$$19x+5\left(-165\right)=30$$

Multiply 5 times -165.

$$19x-825=30$$

Add 825 to both sides of the equation.

$$19x=855$$

Divide both sides by 19.

$$x=45$$

The system is now solved.

$$x=45,y=-165$$