Solve x^2+8x+7=

$$x ^ { 2 } + 8 x + 7 =$$

Factor

$$\left(x+1\right)\left(x+7\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx+7. To find a and b, set up a system to be solved.

$$a+b=8 ab=1\times 7=7$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. The only such pair is the system solution.

$$a=1 b=7$$

Rewrite x^{2}+8x+7 as \left(x^{2}+x\right)+\left(7x+7\right).

$$\left(x^{2}+x\right)+\left(7x+7\right)$$

Factor out x in the first and 7 in the second group.

$$x\left(x+1\right)+7\left(x+1\right)$$

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

$$\left(x+1\right)\left(x+7\right)$$

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.

$$x^{2}+8x+7=0$$

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.

$$x=\frac{-8±\sqrt{8^{2}-4\times 7}}{2}$$

Square 8.

$$x=\frac{-8±\sqrt{64-4\times 7}}{2}$$

Multiply -4 times 7.

$$x=\frac{-8±\sqrt{64-28}}{2}$$

Add 64 to -28.

$$x=\frac{-8±\sqrt{36}}{2}$$

Take the square root of 36.

$$x=\frac{-8±6}{2}$$

Now solve the equation x=\frac{-8±6}{2} when ± is plus. Add -8 to 6.

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

Divide -2 by 2.

$$x=-1$$

Now solve the equation x=\frac{-8±6}{2} when ± is minus. Subtract 6 from -8.

$$x=-\frac{14}{2}$$

Divide -14 by 2.

$$x=-7$$

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 -7 for x_{2}.

$$x^{2}+8x+7=\left(x-\left(-1\right)\right)\left(x-\left(-7\right)\right)$$

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

$$x^{2}+8x+7=\left(x+1\right)\left(x+7\right)$$

Steps Using Direct Factoring Method

Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form x^2+Bx+C=0.

$$x ^ 2 +8x +7 = 0$$

Let r and s be the factors for the quadratic equation such that x^2+Bx+C=(x−r)(x−s) where sum of factors (r+s)=−B and the product of factors rs = C

$$r + s = -8 rs = 7$$

Two numbers r and s sum up to -8 exactly when the average of the two numbers is \frac{1}{2}*-8 = -4. You can also see that the midpoint of r and s corresponds to the axis of symmetry of the parabola represented by the quadratic equation y=x^2+Bx+C. The values of r and s are equidistant from the center by an unknown quantity u. Express r and s with respect to variable u. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>

$$r = -4 - u s = -4 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 7

$$(-4 - u) (-4 + u) = 7$$

Simplify by expanding (a -b) (a + b) = a^2 – b^2

$$16 - u^2 = 7$$

Simplify the expression by subtracting 16 on both sides

$$-u^2 = 7-16 = -9$$

Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u

$$u^2 = 9 u = \pm\sqrt{9} = \pm 3$$

The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.

$$r =-4 - 3 = -7 s = -4 + 3 = -1$$

Evaluate

$$\left(x+1\right)\left(x+7\right)$$