Solve {l}{1/2(3a+3b)=3}{1/3(a-6b)=4}

$$\left\{ \begin{array} { l } { \frac { 1 } { 2 } ( 3 a + 3 b ) = 3 } \\ { \frac { 1 } { 3 } ( a - 6 b ) = 4 } \end{array} \right.$$

Solve for a, b

$$a = \frac{24}{7} = 3\frac{3}{7} \approx 3.428571429$$

$$b = -\frac{10}{7} = -1\frac{3}{7} \approx -1.428571429$$

Steps Using Substitution

Consider the first equation. Multiply both sides by 2, the reciprocal of \frac{1}{2}.

$$3a+3b=3\times 2$$

Multiply 3 and 2 to get 6.

$$3a+3b=6$$

Consider the second equation. Multiply both sides by 3, the reciprocal of \frac{1}{3}.

$$a-6b=4\times 3$$

Multiply 4 and 3 to get 12.

$$a-6b=12$$

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.

$$3a+3b=6,a-6b=12$$

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

$$3a+3b=6$$

Subtract 3b from both sides of the equation.

$$3a=-3b+6$$

Divide both sides by 3.

$$a=\frac{1}{3}\left(-3b+6\right)$$

Multiply \frac{1}{3} times -3b+6.

$$a=-b+2$$

Substitute -b+2 for a in the other equation, a-6b=12.

$$-b+2-6b=12$$

Add -b to -6b.

$$-7b+2=12$$

Subtract 2 from both sides of the equation.

$$-7b=10$$

Divide both sides by -7.

$$b=-\frac{10}{7}$$

Substitute -\frac{10}{7} for b in a=-b+2. Because the resulting equation contains only one variable, you can solve for a directly.

$$a=-\left(-\frac{10}{7}\right)+2$$

Multiply -1 times -\frac{10}{7}.

$$a=\frac{10}{7}+2$$

Add 2 to \frac{10}{7}.

$$a=\frac{24}{7}$$

The system is now solved.

$$a=\frac{24}{7},b=-\frac{10}{7}$$

Steps Using Matrices

Consider the first equation. Multiply both sides by 2, the reciprocal of \frac{1}{2}.

$$3a+3b=3\times 2$$

Multiply 3 and 2 to get 6.

$$3a+3b=6$$

Consider the second equation. Multiply both sides by 3, the reciprocal of \frac{1}{3}.

$$a-6b=4\times 3$$

Multiply 4 and 3 to get 12.

$$a-6b=12$$

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

$$3a+3b=6,a-6b=12$$

Write the equations in matrix form.

$$\left(\begin{matrix}3&3\\1&-6\end{matrix}\right)\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}6\\12\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}3&3\\1&-6\end{matrix}\right).

$$inverse(\left(\begin{matrix}3&3\\1&-6\end{matrix}\right))\left(\begin{matrix}3&3\\1&-6\end{matrix}\right)\left(\begin{matrix}a\\b\end{matrix}\right)=inverse(\left(\begin{matrix}3&3\\1&-6\end{matrix}\right))\left(\begin{matrix}6\\12\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}a\\b\end{matrix}\right)=inverse(\left(\begin{matrix}3&3\\1&-6\end{matrix}\right))\left(\begin{matrix}6\\12\end{matrix}\right)$$

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

$$\left(\begin{matrix}a\\b\end{matrix}\right)=inverse(\left(\begin{matrix}3&3\\1&-6\end{matrix}\right))\left(\begin{matrix}6\\12\end{matrix}\right)$$

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.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}-\frac{6}{3\left(-6\right)-3}&-\frac{3}{3\left(-6\right)-3}\\-\frac{1}{3\left(-6\right)-3}&\frac{3}{3\left(-6\right)-3}\end{matrix}\right)\left(\begin{matrix}6\\12\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}\frac{2}{7}&\frac{1}{7}\\\frac{1}{21}&-\frac{1}{7}\end{matrix}\right)\left(\begin{matrix}6\\12\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}\frac{2}{7}\times 6+\frac{1}{7}\times 12\\\frac{1}{21}\times 6-\frac{1}{7}\times 12\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}a\\b\end{matrix}\right)=\left(\begin{matrix}\frac{24}{7}\\-\frac{10}{7}\end{matrix}\right)$$

Extract the matrix elements a and b.

$$a=\frac{24}{7},b=-\frac{10}{7}$$

Steps Using Elimination

Consider the first equation. Multiply both sides by 2, the reciprocal of \frac{1}{2}.

$$3a+3b=3\times 2$$

Multiply 3 and 2 to get 6.

$$3a+3b=6$$

Consider the second equation. Multiply both sides by 3, the reciprocal of \frac{1}{3}.

$$a-6b=4\times 3$$

Multiply 4 and 3 to get 12.

$$a-6b=12$$

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.

$$3a+3b=6,a-6b=12$$

To make 3a and a equal, multiply all terms on each side of the first equation by 1 and all terms on each side of the second by 3.

$$3a+3b=6,3a+3\left(-6\right)b=3\times 12$$

Simplify.

$$3a+3b=6,3a-18b=36$$

Subtract 3a-18b=36 from 3a+3b=6 by subtracting like terms on each side of the equal sign.

$$3a-3a+3b+18b=6-36$$

Add 3a to -3a. Terms 3a and -3a cancel out, leaving an equation with only one variable that can be solved.

$$3b+18b=6-36$$

Add 3b to 18b.

$$21b=6-36$$

Add 6 to -36.

$$21b=-30$$

Divide both sides by 21.

$$b=-\frac{10}{7}$$

Substitute -\frac{10}{7} for b in a-6b=12. Because the resulting equation contains only one variable, you can solve for a directly.

$$a-6\left(-\frac{10}{7}\right)=12$$

Multiply -6 times -\frac{10}{7}.

$$a+\frac{60}{7}=12$$

Subtract \frac{60}{7} from both sides of the equation.

$$a=\frac{24}{7}$$

The system is now solved.

$$a=\frac{24}{7},b=-\frac{10}{7}$$