Solve {l}{4a+2b=34}{16a+3b=91}

$$\left. \begin{array}{l}{ 4 a + 2 b = 34 }\\{ 16 a + 3 b = 91 }\end{array} \right.$$

Solve for a, b

$$a=4$$

$$b=9$$

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.

$$4a+2b=34,16a+3b=91$$

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

$$4a+2b=34$$

Subtract 2b from both sides of the equation.

$$4a=-2b+34$$

Divide both sides by 4.

$$a=\frac{1}{4}\left(-2b+34\right)$$

Multiply \frac{1}{4} times -2b+34.

$$a=-\frac{1}{2}b+\frac{17}{2}$$

Substitute \frac{-b+17}{2} for a in the other equation, 16a+3b=91.

$$16\left(-\frac{1}{2}b+\frac{17}{2}\right)+3b=91$$

Multiply 16 times \frac{-b+17}{2}.

$$-8b+136+3b=91$$

Add -8b to 3b.

$$-5b+136=91$$

Subtract 136 from both sides of the equation.

$$-5b=-45$$

Divide both sides by -5.

$$b=9$$

Substitute 9 for b in a=-\frac{1}{2}b+\frac{17}{2}. Because the resulting equation contains only one variable, you can solve for a directly.

$$a=-\frac{1}{2}\times 9+\frac{17}{2}$$

Multiply -\frac{1}{2} times 9.

$$a=\frac{-9+17}{2}$$

Add \frac{17}{2} to -\frac{9}{2} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$a=4$$

The system is now solved.

$$a=4,b=9$$

Steps Using Matrices

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

$$4a+2b=34,16a+3b=91$$

Write the equations in matrix form.

$$\left(\begin{matrix}4&2\\16&3\end{matrix}\right)\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}34\\91\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}4&2\\16&3\end{matrix}\right).

$$inverse(\left(\begin{matrix}4&2\\16&3\end{matrix}\right))\left(\begin{matrix}4&2\\16&3\end{matrix}\right)\left(\begin{matrix}a\\b\end{matrix}\right)=inverse(\left(\begin{matrix}4&2\\16&3\end{matrix}\right))\left(\begin{matrix}34\\91\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}a\\b\end{matrix}\right)=inverse(\left(\begin{matrix}4&2\\16&3\end{matrix}\right))\left(\begin{matrix}34\\91\end{matrix}\right)$$

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

$$\left(\begin{matrix}a\\b\end{matrix}\right)=inverse(\left(\begin{matrix}4&2\\16&3\end{matrix}\right))\left(\begin{matrix}34\\91\end{matrix}\right)$$

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.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}\frac{3}{4\times 3-2\times 16}&-\frac{2}{4\times 3-2\times 16}\\-\frac{16}{4\times 3-2\times 16}&\frac{4}{4\times 3-2\times 16}\end{matrix}\right)\left(\begin{matrix}34\\91\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}-\frac{3}{20}&\frac{1}{10}\\\frac{4}{5}&-\frac{1}{5}\end{matrix}\right)\left(\begin{matrix}34\\91\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}-\frac{3}{20}\times 34+\frac{1}{10}\times 91\\\frac{4}{5}\times 34-\frac{1}{5}\times 91\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}4\\9\end{matrix}\right)$$

Extract the matrix elements a and b.

$$a=4,b=9$$

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.

$$4a+2b=34,16a+3b=91$$

To make 4a and 16a equal, multiply all terms on each side of the first equation by 16 and all terms on each side of the second by 4.

$$16\times 4a+16\times 2b=16\times 34,4\times 16a+4\times 3b=4\times 91$$

Simplify.

$$64a+32b=544,64a+12b=364$$

Subtract 64a+12b=364 from 64a+32b=544 by subtracting like terms on each side of the equal sign.

$$64a-64a+32b-12b=544-364$$

Add 64a to -64a. Terms 64a and -64a cancel out, leaving an equation with only one variable that can be solved.

$$32b-12b=544-364$$

Add 32b to -12b.

$$20b=544-364$$

Add 544 to -364.

$$20b=180$$

Divide both sides by 20.

$$b=9$$

Substitute 9 for b in 16a+3b=91. Because the resulting equation contains only one variable, you can solve for a directly.

$$16a+3\times 9=91$$

Multiply 3 times 9.

$$16a+27=91$$

Subtract 27 from both sides of the equation.

$$16a=64$$

Divide both sides by 16.

$$a=4$$

The system is now solved.

$$a=4,b=9$$