Solve 4x^2+17x+15

$$4 x ^ { 2 } + 17 x + 15$$

Factor

$$\left(x+3\right)\left(4x+5\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 4x^{2}+ax+bx+15. To find a and b, set up a system to be solved.

$$a+b=17 ab=4\times 15=60$$

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 60.

$$1,60 2,30 3,20 4,15 5,12 6,10$$

Calculate the sum for each pair.

$$1+60=61 2+30=32 3+20=23 4+15=19 5+12=17 6+10=16$$

The solution is the pair that gives sum 17.

$$a=5 b=12$$

Rewrite 4x^{2}+17x+15 as \left(4x^{2}+5x\right)+\left(12x+15\right).

$$\left(4x^{2}+5x\right)+\left(12x+15\right)$$

Factor out x in the first and 3 in the second group.

$$x\left(4x+5\right)+3\left(4x+5\right)$$

Factor out common term 4x+5 by using distributive property.

$$\left(4x+5\right)\left(x+3\right)$$

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.

$$4x^{2}+17x+15=0$$

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.

$$x=\frac{-17±\sqrt{17^{2}-4\times 4\times 15}}{2\times 4}$$

Square 17.

$$x=\frac{-17±\sqrt{289-4\times 4\times 15}}{2\times 4}$$

Multiply -4 times 4.

$$x=\frac{-17±\sqrt{289-16\times 15}}{2\times 4}$$

Multiply -16 times 15.

$$x=\frac{-17±\sqrt{289-240}}{2\times 4}$$

Add 289 to -240.

$$x=\frac{-17±\sqrt{49}}{2\times 4}$$

Take the square root of 49.

$$x=\frac{-17±7}{2\times 4}$$

Multiply 2 times 4.

$$x=\frac{-17±7}{8}$$

Now solve the equation x=\frac{-17±7}{8} when ± is plus. Add -17 to 7.

$$x=-\frac{10}{8}$$

Reduce the fraction \frac{-10}{8} to lowest terms by extracting and canceling out 2.

$$x=-\frac{5}{4}$$

Now solve the equation x=\frac{-17±7}{8} when ± is minus. Subtract 7 from -17.

$$x=-\frac{24}{8}$$

Divide -24 by 8.

$$x=-3$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -\frac{5}{4} for x_{1} and -3 for x_{2}.

$$4x^{2}+17x+15=4\left(x-\left(-\frac{5}{4}\right)\right)\left(x-\left(-3\right)\right)$$

Simplify all the expressions of the form p-\left(-q\right) to p+q.

$$4x^{2}+17x+15=4\left(x+\frac{5}{4}\right)\left(x+3\right)$$

Add \frac{5}{4} to x by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$4x^{2}+17x+15=4\times \frac{4x+5}{4}\left(x+3\right)$$

Cancel out 4, the greatest common factor in 4 and 4.

$$4x^{2}+17x+15=\left(4x+5\right)\left(x+3\right)$$

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.This is achieved by dividing both sides of the equation by 4

$$x ^ 2 +\frac{17}{4}x +\frac{15}{4} = 0$$

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

$$r + s = -\frac{17}{4} rs = \frac{15}{4}$$

Two numbers r and s sum up to -\frac{17}{4} exactly when the average of the two numbers is \frac{1}{2}*-\frac{17}{4} = -\frac{17}{8}. 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>

$$r = -\frac{17}{8} - u s = -\frac{17}{8} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = \frac{15}{4}

$$(-\frac{17}{8} - u) (-\frac{17}{8} + u) = \frac{15}{4}$$

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

$$\frac{289}{64} - u^2 = \frac{15}{4}$$

Simplify the expression by subtracting \frac{289}{64} on both sides

$$-u^2 = \frac{15}{4}-\frac{289}{64} = -\frac{49}{64}$$

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

$$u^2 = \frac{49}{64} u = \pm\sqrt{\frac{49}{64}} = \pm \frac{7}{8}$$

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

$$r =-\frac{17}{8} - \frac{7}{8} = -3 s = -\frac{17}{8} + \frac{7}{8} = -1.250$$

Evaluate

$$\left(x+3\right)\left(4x+5\right)$$