Solve -5t^2+30t+8

$$- 5 t ^ { 2 } + 30 t + 8$$

Factor

$$-5\left(t-\left(-\frac{\sqrt{265}}{5}+3\right)\right)\left(t-\left(\frac{\sqrt{265}}{5}+3\right)\right)$$

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.

$$-5t^{2}+30t+8=0$$

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.

$$t=\frac{-30±\sqrt{30^{2}-4\left(-5\right)\times 8}}{2\left(-5\right)}$$

Square 30.

$$t=\frac{-30±\sqrt{900-4\left(-5\right)\times 8}}{2\left(-5\right)}$$

Multiply -4 times -5.

$$t=\frac{-30±\sqrt{900+20\times 8}}{2\left(-5\right)}$$

Multiply 20 times 8.

$$t=\frac{-30±\sqrt{900+160}}{2\left(-5\right)}$$

Add 900 to 160.

$$t=\frac{-30±\sqrt{1060}}{2\left(-5\right)}$$

Take the square root of 1060.

$$t=\frac{-30±2\sqrt{265}}{2\left(-5\right)}$$

Multiply 2 times -5.

$$t=\frac{-30±2\sqrt{265}}{-10}$$

Now solve the equation t=\frac{-30±2\sqrt{265}}{-10} when ± is plus. Add -30 to 2\sqrt{265}.

$$t=\frac{2\sqrt{265}-30}{-10}$$

Divide -30+2\sqrt{265} by -10.

$$t=-\frac{\sqrt{265}}{5}+3$$

Now solve the equation t=\frac{-30±2\sqrt{265}}{-10} when ± is minus. Subtract 2\sqrt{265} from -30.

$$t=\frac{-2\sqrt{265}-30}{-10}$$

Divide -30-2\sqrt{265} by -10.

$$t=\frac{\sqrt{265}}{5}+3$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 3-\frac{\sqrt{265}}{5} for x_{1} and 3+\frac{\sqrt{265}}{5} for x_{2}.

$$-5t^{2}+30t+8=-5\left(t-\left(-\frac{\sqrt{265}}{5}+3\right)\right)\left(t-\left(\frac{\sqrt{265}}{5}+3\right)\right)$$

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.

$$x ^ 2 -6x -\frac{8}{5} = 0$$

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

$$r + s = 6 rs = -\frac{8}{5}$$

Two numbers r and s sum up to 6 exactly when the average of the two numbers is \frac{1}{2}*6 = 3. 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>

$$r = 3 - u s = 3 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{8}{5}

$$(3 - u) (3 + u) = -\frac{8}{5}$$

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

$$9 - u^2 = -\frac{8}{5}$$

Simplify the expression by subtracting 9 on both sides

$$-u^2 = -\frac{8}{5}-9 = -\frac{53}{5}$$

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

$$u^2 = \frac{53}{5} u = \pm\sqrt{\frac{53}{5}} = \pm \frac{\sqrt{53}}{\sqrt{5}}$$

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

$$r =3 - \frac{\sqrt{53}}{\sqrt{5}} = -0.256 s = 3 + \frac{\sqrt{53}}{\sqrt{5}} = 6.256$$

Evaluate

$$8+30t-5t^{2}$$