Solve y^2+3y-21

$$y ^ { 2 } + 3 y - 21$$

Factor

$$\left(y-\frac{-\sqrt{93}-3}{2}\right)\left(y-\frac{\sqrt{93}-3}{2}\right)$$

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.

$$y^{2}+3y-21=0$$

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.

$$y=\frac{-3±\sqrt{3^{2}-4\left(-21\right)}}{2}$$

Square 3.

$$y=\frac{-3±\sqrt{9-4\left(-21\right)}}{2}$$

Multiply -4 times -21.

$$y=\frac{-3±\sqrt{9+84}}{2}$$

Add 9 to 84.

$$y=\frac{-3±\sqrt{93}}{2}$$

Now solve the equation y=\frac{-3±\sqrt{93}}{2} when ± is plus. Add -3 to \sqrt{93}.

$$y=\frac{\sqrt{93}-3}{2}$$

Now solve the equation y=\frac{-3±\sqrt{93}}{2} when ± is minus. Subtract \sqrt{93} from -3.

$$y=\frac{-\sqrt{93}-3}{2}$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute \frac{-3+\sqrt{93}}{2} for x_{1} and \frac{-3-\sqrt{93}}{2} for x_{2}.

$$y^{2}+3y-21=\left(y-\frac{\sqrt{93}-3}{2}\right)\left(y-\frac{-\sqrt{93}-3}{2}\right)$$

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.

$$x ^ 2 +3x -21 = 0$$

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

$$r + s = -3 rs = -21$$

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

$$r = -\frac{3}{2} - u s = -\frac{3}{2} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -21

$$(-\frac{3}{2} - u) (-\frac{3}{2} + u) = -21$$

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

$$\frac{9}{4} - u^2 = -21$$

Simplify the expression by subtracting \frac{9}{4} on both sides

$$-u^2 = -21-\frac{9}{4} = -\frac{93}{4}$$

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

$$u^2 = \frac{93}{4} u = \pm\sqrt{\frac{93}{4}} = \pm \frac{\sqrt{93}}{2}$$

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

$$r =-\frac{3}{2} - \frac{\sqrt{93}}{2} = -6.322 s = -\frac{3}{2} + \frac{\sqrt{93}}{2} = 3.322$$

Evaluate

$$y^{2}+3y-21$$