Solve -3x^2+18x-5

$$- 3 x ^ { 2 } + 18 x - 5$$

Factor

$$-3\left(x-\left(-\frac{\sqrt{66}}{3}+3\right)\right)\left(x-\left(\frac{\sqrt{66}}{3}+3\right)\right)$$

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.

$$-3x^{2}+18x-5=0$$

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.

$$x=\frac{-18±\sqrt{18^{2}-4\left(-3\right)\left(-5\right)}}{2\left(-3\right)}$$

Square 18.

$$x=\frac{-18±\sqrt{324-4\left(-3\right)\left(-5\right)}}{2\left(-3\right)}$$

Multiply -4 times -3.

$$x=\frac{-18±\sqrt{324+12\left(-5\right)}}{2\left(-3\right)}$$

Multiply 12 times -5.

$$x=\frac{-18±\sqrt{324-60}}{2\left(-3\right)}$$

Add 324 to -60.

$$x=\frac{-18±\sqrt{264}}{2\left(-3\right)}$$

Take the square root of 264.

$$x=\frac{-18±2\sqrt{66}}{2\left(-3\right)}$$

Multiply 2 times -3.

$$x=\frac{-18±2\sqrt{66}}{-6}$$

Now solve the equation x=\frac{-18±2\sqrt{66}}{-6} when ± is plus. Add -18 to 2\sqrt{66}.

$$x=\frac{2\sqrt{66}-18}{-6}$$

Divide -18+2\sqrt{66} by -6.

$$x=-\frac{\sqrt{66}}{3}+3$$

Now solve the equation x=\frac{-18±2\sqrt{66}}{-6} when ± is minus. Subtract 2\sqrt{66} from -18.

$$x=\frac{-2\sqrt{66}-18}{-6}$$

Divide -18-2\sqrt{66} by -6.

$$x=\frac{\sqrt{66}}{3}+3$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 3-\frac{\sqrt{66}}{3} for x_{1} and 3+\frac{\sqrt{66}}{3} for x_{2}.

$$-3x^{2}+18x-5=-3\left(x-\left(-\frac{\sqrt{66}}{3}+3\right)\right)\left(x-\left(\frac{\sqrt{66}}{3}+3\right)\right)$$

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.

$$x ^ 2 -6x +\frac{5}{3} = 0$$

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

$$r + s = 6 rs = \frac{5}{3}$$

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{5}{3}

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

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

$$9 - u^2 = \frac{5}{3}$$

Simplify the expression by subtracting 9 on both sides

$$-u^2 = \frac{5}{3}-9 = -\frac{22}{3}$$

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

$$u^2 = \frac{22}{3} u = \pm\sqrt{\frac{22}{3}} = \pm \frac{\sqrt{22}}{\sqrt{3}}$$

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

$$r =3 - \frac{\sqrt{22}}{\sqrt{3}} = 0.292 s = 3 + \frac{\sqrt{22}}{\sqrt{3}} = 5.708$$

Evaluate

$$-3x^{2}+18x-5$$