Solve -x^2+8x+15

$$- x ^ { 2 } + 8 x + 15$$

Factor

$$-\left(x-\left(4-\sqrt{31}\right)\right)\left(x-\left(\sqrt{31}+4\right)\right)$$

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.

$$-x^{2}+8x+15=0$$

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.

$$x=\frac{-8±\sqrt{8^{2}-4\left(-1\right)\times 15}}{2\left(-1\right)}$$

Square 8.

$$x=\frac{-8±\sqrt{64-4\left(-1\right)\times 15}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-8±\sqrt{64+4\times 15}}{2\left(-1\right)}$$

Multiply 4 times 15.

$$x=\frac{-8±\sqrt{64+60}}{2\left(-1\right)}$$

Add 64 to 60.

$$x=\frac{-8±\sqrt{124}}{2\left(-1\right)}$$

Take the square root of 124.

$$x=\frac{-8±2\sqrt{31}}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{-8±2\sqrt{31}}{-2}$$

Now solve the equation x=\frac{-8±2\sqrt{31}}{-2} when ± is plus. Add -8 to 2\sqrt{31}.

$$x=\frac{2\sqrt{31}-8}{-2}$$

Divide -8+2\sqrt{31} by -2.

$$x=4-\sqrt{31}$$

Now solve the equation x=\frac{-8±2\sqrt{31}}{-2} when ± is minus. Subtract 2\sqrt{31} from -8.

$$x=\frac{-2\sqrt{31}-8}{-2}$$

Divide -8-2\sqrt{31} by -2.

$$x=\sqrt{31}+4$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 4-\sqrt{31} for x_{1} and 4+\sqrt{31} for x_{2}.

$$-x^{2}+8x+15=-\left(x-\left(4-\sqrt{31}\right)\right)\left(x-\left(\sqrt{31}+4\right)\right)$$

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.

$$x ^ 2 -8x -15 = 0$$

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

$$r + s = 8 rs = -15$$

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 = -15

$$(4 - u) (4 + u) = -15$$

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

$$16 - u^2 = -15$$

Simplify the expression by subtracting 16 on both sides

$$-u^2 = -15-16 = -31$$

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

$$u^2 = 31 u = \pm\sqrt{31} = \pm \sqrt{31}$$

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

$$r =4 - \sqrt{31} = -1.568 s = 4 + \sqrt{31} = 9.568$$

Evaluate

$$15+8x-x^{2}$$