Solve 18k^2+25k+8=0

$$18 k ^ { 2 } + 25 k + 8 = 0$$

Solve for k

$$k=-\frac{8}{9}\approx -0.888888889$$

$$k=-\frac{1}{2}=-0.5$$

Steps Using Factoring By Grouping

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as 18k^{2}+ak+bk+8. To find a and b, set up a system to be solved.

$$a+b=25 ab=18\times 8=144$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. List all such integer pairs that give product 144.

$$1,144 2,72 3,48 4,36 6,24 8,18 9,16 12,12$$

Calculate the sum for each pair.

$$1+144=145 2+72=74 3+48=51 4+36=40 6+24=30 8+18=26 9+16=25 12+12=24$$

The solution is the pair that gives sum 25.

$$a=9 b=16$$

Rewrite 18k^{2}+25k+8 as \left(18k^{2}+9k\right)+\left(16k+8\right).

$$\left(18k^{2}+9k\right)+\left(16k+8\right)$$

Factor out 9k in the first and 8 in the second group.

$$9k\left(2k+1\right)+8\left(2k+1\right)$$

Factor out common term 2k+1 by using distributive property.

$$\left(2k+1\right)\left(9k+8\right)$$

To find equation solutions, solve 2k+1=0 and 9k+8=0.

$$k=-\frac{1}{2} k=-\frac{8}{9}$$

Steps Using the Quadratic Formula

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.

$$18k^{2}+25k+8=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 18 for a, 25 for b, and 8 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$k=\frac{-25±\sqrt{25^{2}-4\times 18\times 8}}{2\times 18}$$

Square 25.

$$k=\frac{-25±\sqrt{625-4\times 18\times 8}}{2\times 18}$$

Multiply -4 times 18.

$$k=\frac{-25±\sqrt{625-72\times 8}}{2\times 18}$$

Multiply -72 times 8.

$$k=\frac{-25±\sqrt{625-576}}{2\times 18}$$

Add 625 to -576.

$$k=\frac{-25±\sqrt{49}}{2\times 18}$$

Take the square root of 49.

$$k=\frac{-25±7}{2\times 18}$$

Multiply 2 times 18.

$$k=\frac{-25±7}{36}$$

Now solve the equation k=\frac{-25±7}{36} when ± is plus. Add -25 to 7.

$$k=-\frac{18}{36}$$

Reduce the fraction \frac{-18}{36} to lowest terms by extracting and canceling out 18.

$$k=-\frac{1}{2}$$

Now solve the equation k=\frac{-25±7}{36} when ± is minus. Subtract 7 from -25.

$$k=-\frac{32}{36}$$

Reduce the fraction \frac{-32}{36} to lowest terms by extracting and canceling out 4.

$$k=-\frac{8}{9}$$

The equation is now solved.

$$k=-\frac{1}{2} k=-\frac{8}{9}$$

Steps for Completing the Square

Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.

$$18k^{2}+25k+8=0$$

Subtract 8 from both sides of the equation.

$$18k^{2}+25k+8-8=-8$$

Subtracting 8 from itself leaves 0.

$$18k^{2}+25k=-8$$

Divide both sides by 18.

$$\frac{18k^{2}+25k}{18}=-\frac{8}{18}$$

Dividing by 18 undoes the multiplication by 18.

$$k^{2}+\frac{25}{18}k=-\frac{8}{18}$$

Reduce the fraction \frac{-8}{18} to lowest terms by extracting and canceling out 2.

$$k^{2}+\frac{25}{18}k=-\frac{4}{9}$$

Divide \frac{25}{18}, the coefficient of the x term, by 2 to get \frac{25}{36}. Then add the square of \frac{25}{36} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$k^{2}+\frac{25}{18}k+\left(\frac{25}{36}\right)^{2}=-\frac{4}{9}+\left(\frac{25}{36}\right)^{2}$$

Square \frac{25}{36} by squaring both the numerator and the denominator of the fraction.

$$k^{2}+\frac{25}{18}k+\frac{625}{1296}=-\frac{4}{9}+\frac{625}{1296}$$

Add -\frac{4}{9} to \frac{625}{1296} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$k^{2}+\frac{25}{18}k+\frac{625}{1296}=\frac{49}{1296}$$

Factor k^{2}+\frac{25}{18}k+\frac{625}{1296}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(k+\frac{25}{36}\right)^{2}=\frac{49}{1296}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(k+\frac{25}{36}\right)^{2}}=\sqrt{\frac{49}{1296}}$$

Simplify.

$$k+\frac{25}{36}=\frac{7}{36} k+\frac{25}{36}=-\frac{7}{36}$$

Subtract \frac{25}{36} from both sides of the equation.

$$k=-\frac{1}{2} k=-\frac{8}{9}$$

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.This is achieved by dividing both sides of the equation by 18

$$x ^ 2 +\frac{25}{18}x +\frac{4}{9} = 0$$

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

$$r + s = -\frac{25}{18} rs = \frac{4}{9}$$

Two numbers r and s sum up to -\frac{25}{18} exactly when the average of the two numbers is \frac{1}{2}*-\frac{25}{18} = -\frac{25}{36}. 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>

$$r = -\frac{25}{36} - u s = -\frac{25}{36} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = \frac{4}{9}

$$(-\frac{25}{36} - u) (-\frac{25}{36} + u) = \frac{4}{9}$$

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

$$\frac{625}{1296} - u^2 = \frac{4}{9}$$

Simplify the expression by subtracting \frac{625}{1296} on both sides

$$-u^2 = \frac{4}{9}-\frac{625}{1296} = -\frac{49}{1296}$$

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

$$u^2 = \frac{49}{1296} u = \pm\sqrt{\frac{49}{1296}} = \pm \frac{7}{36}$$

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

$$r =-\frac{25}{36} - \frac{7}{36} = -0.889 s = -\frac{25}{36} + \frac{7}{36} = -0.500$$