Solve -8r^2+11r-6

$$- 8 r ^ { 2 } + 11 r - 6$$

Evaluate

$$-8r^{2}+11r-6$$

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.

$$x ^ 2 -\frac{11}{8}x +\frac{3}{4} = 0$$

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

$$r + s = \frac{11}{8} rs = \frac{3}{4}$$

Two numbers r and s sum up to \frac{11}{8} exactly when the average of the two numbers is \frac{1}{2}*\frac{11}{8} = \frac{11}{16}. 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>

$$r = \frac{11}{16} - u s = \frac{11}{16} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = \frac{3}{4}

$$(\frac{11}{16} - u) (\frac{11}{16} + u) = \frac{3}{4}$$

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

$$\frac{121}{256} - u^2 = \frac{3}{4}$$

Simplify the expression by subtracting \frac{121}{256} on both sides

$$-u^2 = \frac{3}{4}-\frac{121}{256} = \frac{71}{256}$$

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

$$u^2 = -\frac{71}{256} u = \pm\sqrt{-\frac{71}{256}} = \pm \frac{\sqrt{71}}{16}i$$

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

$$r =\frac{11}{16} - \frac{\sqrt{71}}{16}i = 0.688 - 0.527i s = \frac{11}{16} + \frac{\sqrt{71}}{16}i = 0.688 + 0.527i$$

Differentiate w.r.t. r

$$11-16r$$

Steps Using Derivative Rule for Sum

The derivative of a polynomial is the sum of the derivatives of its terms. The derivative of a constant term is 0. The derivative of ax^{n} is nax^{n-1}.

$$2\left(-8\right)r^{2-1}+11r^{1-1}$$

Multiply 2 times -8.

$$-16r^{2-1}+11r^{1-1}$$

Subtract 1 from 2.

$$-16r^{1}+11r^{1-1}$$

Subtract 1 from 1.

$$-16r^{1}+11r^{0}$$

For any term t, t^{1}=t.

$$-16r+11r^{0}$$

For any term t except 0, t^{0}=1.

$$-16r+11\times 1$$

For any term t, t\times 1=t and 1t=t.

$$-16r+11$$