Solve 4y^2+9y+5

$$4 y ^ { 2 } + 9 y + 5$$

Factor

$$\left(y+1\right)\left(4y+5\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 4y^{2}+ay+by+5. To find a and b, set up a system to be solved.

$$a+b=9 ab=4\times 5=20$$

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 20.

$$1,20 2,10 4,5$$

Calculate the sum for each pair.

$$1+20=21 2+10=12 4+5=9$$

The solution is the pair that gives sum 9.

$$a=4 b=5$$

Rewrite 4y^{2}+9y+5 as \left(4y^{2}+4y\right)+\left(5y+5\right).

$$\left(4y^{2}+4y\right)+\left(5y+5\right)$$

Factor out 4y in the first and 5 in the second group.

$$4y\left(y+1\right)+5\left(y+1\right)$$

Factor out common term y+1 by using distributive property.

$$\left(y+1\right)\left(4y+5\right)$$

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.

$$4y^{2}+9y+5=0$$

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.

$$y=\frac{-9±\sqrt{9^{2}-4\times 4\times 5}}{2\times 4}$$

Square 9.

$$y=\frac{-9±\sqrt{81-4\times 4\times 5}}{2\times 4}$$

Multiply -4 times 4.

$$y=\frac{-9±\sqrt{81-16\times 5}}{2\times 4}$$

Multiply -16 times 5.

$$y=\frac{-9±\sqrt{81-80}}{2\times 4}$$

Add 81 to -80.

$$y=\frac{-9±\sqrt{1}}{2\times 4}$$

Take the square root of 1.

$$y=\frac{-9±1}{2\times 4}$$

Multiply 2 times 4.

$$y=\frac{-9±1}{8}$$

Now solve the equation y=\frac{-9±1}{8} when ± is plus. Add -9 to 1.

$$y=-\frac{8}{8}$$

Divide -8 by 8.

$$y=-1$$

Now solve the equation y=\frac{-9±1}{8} when ± is minus. Subtract 1 from -9.

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

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

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

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

$$4y^{2}+9y+5=4\left(y-\left(-1\right)\right)\left(y-\left(-\frac{5}{4}\right)\right)$$

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

$$4y^{2}+9y+5=4\left(y+1\right)\left(y+\frac{5}{4}\right)$$

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

$$4y^{2}+9y+5=4\left(y+1\right)\times \frac{4y+5}{4}$$

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

$$4y^{2}+9y+5=\left(y+1\right)\left(4y+5\right)$$

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

$$x ^ 2 +\frac{9}{4}x +\frac{5}{4} = 0$$

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

$$r + s = -\frac{9}{4} rs = \frac{5}{4}$$

Two numbers r and s sum up to -\frac{9}{4} exactly when the average of the two numbers is \frac{1}{2}*-\frac{9}{4} = -\frac{9}{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>

$$r = -\frac{9}{8} - u s = -\frac{9}{8} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = \frac{5}{4}

$$(-\frac{9}{8} - u) (-\frac{9}{8} + u) = \frac{5}{4}$$

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

$$\frac{81}{64} - u^2 = \frac{5}{4}$$

Simplify the expression by subtracting \frac{81}{64} on both sides

$$-u^2 = \frac{5}{4}-\frac{81}{64} = -\frac{1}{64}$$

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

$$u^2 = \frac{1}{64} u = \pm\sqrt{\frac{1}{64}} = \pm \frac{1}{8}$$

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

$$r =-\frac{9}{8} - \frac{1}{8} = -1.250 s = -\frac{9}{8} + \frac{1}{8} = -1$$

Evaluate

$$\left(y+1\right)\left(4y+5\right)$$