Solve y^2-2y-63

$$y ^ { 2 } - 2 y - 63$$

Factor

$$\left(y-9\right)\left(y+7\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as y^{2}+ay+by-63. To find a and b, set up a system to be solved.

$$a+b=-2 ab=1\left(-63\right)=-63$$

Since ab is negative, a and b have the opposite signs. Since a+b is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product -63.

$$1,-63 3,-21 7,-9$$

Calculate the sum for each pair.

$$1-63=-62 3-21=-18 7-9=-2$$

The solution is the pair that gives sum -2.

$$a=-9 b=7$$

Rewrite y^{2}-2y-63 as \left(y^{2}-9y\right)+\left(7y-63\right).

$$\left(y^{2}-9y\right)+\left(7y-63\right)$$

Factor out y in the first and 7 in the second group.

$$y\left(y-9\right)+7\left(y-9\right)$$

Factor out common term y-9 by using distributive property.

$$\left(y-9\right)\left(y+7\right)$$

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.

$$y^{2}-2y-63=0$$

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.

$$y=\frac{-\left(-2\right)±\sqrt{\left(-2\right)^{2}-4\left(-63\right)}}{2}$$

Square -2.

$$y=\frac{-\left(-2\right)±\sqrt{4-4\left(-63\right)}}{2}$$

Multiply -4 times -63.

$$y=\frac{-\left(-2\right)±\sqrt{4+252}}{2}$$

Add 4 to 252.

$$y=\frac{-\left(-2\right)±\sqrt{256}}{2}$$

Take the square root of 256.

$$y=\frac{-\left(-2\right)±16}{2}$$

The opposite of -2 is 2.

$$y=\frac{2±16}{2}$$

Now solve the equation y=\frac{2±16}{2} when ± is plus. Add 2 to 16.

$$y=\frac{18}{2}$$

Divide 18 by 2.

$$y=9$$

Now solve the equation y=\frac{2±16}{2} when ± is minus. Subtract 16 from 2.

$$y=-\frac{14}{2}$$

Divide -14 by 2.

$$y=-7$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 9 for x_{1} and -7 for x_{2}.

$$y^{2}-2y-63=\left(y-9\right)\left(y-\left(-7\right)\right)$$

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

$$y^{2}-2y-63=\left(y-9\right)\left(y+7\right)$$

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.

$$x ^ 2 -2x -63 = 0$$

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

$$r + s = 2 rs = -63$$

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

$$r = 1 - u s = 1 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -63

$$(1 - u) (1 + u) = -63$$

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

$$1 - u^2 = -63$$

Simplify the expression by subtracting 1 on both sides

$$-u^2 = -63-1 = -64$$

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

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

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

$$r =1 - 8 = -7 s = 1 + 8 = 9$$

Evaluate

$$\left(y-9\right)\left(y+7\right)$$