Solve x^2+40x-25

$$x ^ { 2 } + 40 x - 25$$

Factor

$$\left(x-\left(-5\sqrt{17}-20\right)\right)\left(x-\left(5\sqrt{17}-20\right)\right)$$

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.

$$x^{2}+40x-25=0$$

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.

$$x=\frac{-40±\sqrt{40^{2}-4\left(-25\right)}}{2}$$

Square 40.

$$x=\frac{-40±\sqrt{1600-4\left(-25\right)}}{2}$$

Multiply -4 times -25.

$$x=\frac{-40±\sqrt{1600+100}}{2}$$

Add 1600 to 100.

$$x=\frac{-40±\sqrt{1700}}{2}$$

Take the square root of 1700.

$$x=\frac{-40±10\sqrt{17}}{2}$$

Now solve the equation x=\frac{-40±10\sqrt{17}}{2} when ± is plus. Add -40 to 10\sqrt{17}.

$$x=\frac{10\sqrt{17}-40}{2}$$

Divide -40+10\sqrt{17} by 2.

$$x=5\sqrt{17}-20$$

Now solve the equation x=\frac{-40±10\sqrt{17}}{2} when ± is minus. Subtract 10\sqrt{17} from -40.

$$x=\frac{-10\sqrt{17}-40}{2}$$

Divide -40-10\sqrt{17} by 2.

$$x=-5\sqrt{17}-20$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -20+5\sqrt{17} for x_{1} and -20-5\sqrt{17} for x_{2}.

$$x^{2}+40x-25=\left(x-\left(5\sqrt{17}-20\right)\right)\left(x-\left(-5\sqrt{17}-20\right)\right)$$

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.

$$x ^ 2 +40x -25 = 0$$

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

$$r + s = -40 rs = -25$$

Two numbers r and s sum up to -40 exactly when the average of the two numbers is \frac{1}{2}*-40 = -20. 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>

$$r = -20 - u s = -20 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -25

$$(-20 - u) (-20 + u) = -25$$

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

$$400 - u^2 = -25$$

Simplify the expression by subtracting 400 on both sides

$$-u^2 = -25-400 = -425$$

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

$$u^2 = 425 u = \pm\sqrt{425} = \pm \sqrt{425}$$

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

$$r =-20 - \sqrt{425} = -40.616 s = -20 + \sqrt{425} = 0.616$$

Evaluate

$$x^{2}+40x-25$$