Solve x^2+10x+9

$${ x }^{ 2 } +10x+9$$

Factor

$$\left(x+1\right)\left(x+9\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx+9. To find a and b, set up a system to be solved.

$$a+b=10 ab=1\times 9=9$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. List all such integer pairs that give product 9.

$$1,9 3,3$$

Calculate the sum for each pair.

$$1+9=10 3+3=6$$

The solution is the pair that gives sum 10.

$$a=1 b=9$$

Rewrite x^{2}+10x+9 as \left(x^{2}+x\right)+\left(9x+9\right).

$$\left(x^{2}+x\right)+\left(9x+9\right)$$

Factor out x in the first and 9 in the second group.

$$x\left(x+1\right)+9\left(x+1\right)$$

Factor out common term x+1 by using distributive property.

$$\left(x+1\right)\left(x+9\right)$$

Steps Using the Quadratic Formula

Quadratic polynomial can be factored using the transformation ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right), where x_{1} and x_{2} are the solutions of the quadratic equation ax^{2}+bx+c=0.

$$x^{2}+10x+9=0$$

All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. The quadratic formula gives two solutions, one when ± is addition and one when it is subtraction.

$$x=\frac{-10±\sqrt{10^{2}-4\times 9}}{2}$$

Square 10.

$$x=\frac{-10±\sqrt{100-4\times 9}}{2}$$

Multiply -4 times 9.

$$x=\frac{-10±\sqrt{100-36}}{2}$$

Add 100 to -36.

$$x=\frac{-10±\sqrt{64}}{2}$$

Take the square root of 64.

$$x=\frac{-10±8}{2}$$

Now solve the equation x=\frac{-10±8}{2} when ± is plus. Add -10 to 8.

$$x=-\frac{2}{2}$$

Divide -2 by 2.

$$x=-1$$

Now solve the equation x=\frac{-10±8}{2} when ± is minus. Subtract 8 from -10.

$$x=-\frac{18}{2}$$

Divide -18 by 2.

$$x=-9$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -1 for x_{1} and -9 for x_{2}.

$$x^{2}+10x+9=\left(x-\left(-1\right)\right)\left(x-\left(-9\right)\right)$$

Simplify all the expressions of the form p-\left(-q\right) to p+q.

$$x^{2}+10x+9=\left(x+1\right)\left(x+9\right)$$

Evaluate

$$\left(x+1\right)\left(x+9\right)$$