Solve 4c^2+3c-10

$$4 c ^ { 2 } + 3 c - 10$$

Factor

$$\left(4c-5\right)\left(c+2\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as 4c^{2}+ac+bc-10. To find a and b, set up a system to be solved.

$$a+b=3 ab=4\left(-10\right)=-40$$

Since ab is negative, a and b have the opposite signs. Since a+b is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product -40.

$$-1,40 -2,20 -4,10 -5,8$$

Calculate the sum for each pair.

$$-1+40=39 -2+20=18 -4+10=6 -5+8=3$$

The solution is the pair that gives sum 3.

$$a=-5 b=8$$

Rewrite 4c^{2}+3c-10 as \left(4c^{2}-5c\right)+\left(8c-10\right).

$$\left(4c^{2}-5c\right)+\left(8c-10\right)$$

Factor out c in the first and 2 in the second group.

$$c\left(4c-5\right)+2\left(4c-5\right)$$

Factor out common term 4c-5 by using distributive property.

$$\left(4c-5\right)\left(c+2\right)$$

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.

$$4c^{2}+3c-10=0$$

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.

$$c=\frac{-3±\sqrt{3^{2}-4\times 4\left(-10\right)}}{2\times 4}$$

Square 3.

$$c=\frac{-3±\sqrt{9-4\times 4\left(-10\right)}}{2\times 4}$$

Multiply -4 times 4.

$$c=\frac{-3±\sqrt{9-16\left(-10\right)}}{2\times 4}$$

Multiply -16 times -10.

$$c=\frac{-3±\sqrt{9+160}}{2\times 4}$$

Add 9 to 160.

$$c=\frac{-3±\sqrt{169}}{2\times 4}$$

Take the square root of 169.

$$c=\frac{-3±13}{2\times 4}$$

Multiply 2 times 4.

$$c=\frac{-3±13}{8}$$

Now solve the equation c=\frac{-3±13}{8} when ± is plus. Add -3 to 13.

$$c=\frac{10}{8}$$

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

$$c=\frac{5}{4}$$

Now solve the equation c=\frac{-3±13}{8} when ± is minus. Subtract 13 from -3.

$$c=-\frac{16}{8}$$

Divide -16 by 8.

$$c=-2$$

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 -2 for x_{2}.

$$4c^{2}+3c-10=4\left(c-\frac{5}{4}\right)\left(c-\left(-2\right)\right)$$

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

$$4c^{2}+3c-10=4\left(c-\frac{5}{4}\right)\left(c+2\right)$$

Subtract \frac{5}{4} from c by finding a common denominator and subtracting the numerators. Then reduce the fraction to lowest terms if possible.

$$4c^{2}+3c-10=4\times \frac{4c-5}{4}\left(c+2\right)$$

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

$$4c^{2}+3c-10=\left(4c-5\right)\left(c+2\right)$$

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

$$x ^ 2 +\frac{3}{4}x -\frac{5}{2} = 0$$

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

$$r + s = -\frac{3}{4} rs = -\frac{5}{2}$$

Two numbers r and s sum up to -\frac{3}{4} exactly when the average of the two numbers is \frac{1}{2}*-\frac{3}{4} = -\frac{3}{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>

$$r = -\frac{3}{8} - u s = -\frac{3}{8} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -\frac{5}{2}

$$(-\frac{3}{8} - u) (-\frac{3}{8} + u) = -\frac{5}{2}$$

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

$$\frac{9}{64} - u^2 = -\frac{5}{2}$$

Simplify the expression by subtracting \frac{9}{64} on both sides

$$-u^2 = -\frac{5}{2}-\frac{9}{64} = -\frac{169}{64}$$

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

$$u^2 = \frac{169}{64} u = \pm\sqrt{\frac{169}{64}} = \pm \frac{13}{8}$$

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

$$r =-\frac{3}{8} - \frac{13}{8} = -2 s = -\frac{3}{8} + \frac{13}{8} = 1.250$$

Evaluate

$$\left(4c-5\right)\left(c+2\right)$$