Solve y^2+6y-7

$$y ^ { 2 } + 6 y - 7$$

Factor

$$\left(y-1\right)\left(y+7\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as y^{2}+ay+by-7. To find a and b, set up a system to be solved.

$$a+b=6 ab=1\left(-7\right)=-7$$

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. The only such pair is the system solution.

$$a=-1 b=7$$

Rewrite y^{2}+6y-7 as \left(y^{2}-y\right)+\left(7y-7\right).

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

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

$$y\left(y-1\right)+7\left(y-1\right)$$

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

$$\left(y-1\right)\left(y+7\right)$$

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.

$$y^{2}+6y-7=0$$

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.

$$y=\frac{-6±\sqrt{6^{2}-4\left(-7\right)}}{2}$$

Square 6.

$$y=\frac{-6±\sqrt{36-4\left(-7\right)}}{2}$$

Multiply -4 times -7.

$$y=\frac{-6±\sqrt{36+28}}{2}$$

Add 36 to 28.

$$y=\frac{-6±\sqrt{64}}{2}$$

Take the square root of 64.

$$y=\frac{-6±8}{2}$$

Now solve the equation y=\frac{-6±8}{2} when ± is plus. Add -6 to 8.

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

Divide 2 by 2.

$$y=1$$

Now solve the equation y=\frac{-6±8}{2} when ± is minus. Subtract 8 from -6.

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

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 1 for x_{1} and -7 for x_{2}.

$$y^{2}+6y-7=\left(y-1\right)\left(y-\left(-7\right)\right)$$

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

$$y^{2}+6y-7=\left(y-1\right)\left(y+7\right)$$

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.

$$x ^ 2 +6x -7 = 0$$

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

$$r + s = -6 rs = -7$$

Two numbers r and s sum up to -6 exactly when the average of the two numbers is \frac{1}{2}*-6 = -3. 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>

$$r = -3 - u s = -3 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -7

$$(-3 - u) (-3 + u) = -7$$

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

$$9 - u^2 = -7$$

Simplify the expression by subtracting 9 on both sides

$$-u^2 = -7-9 = -16$$

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

$$u^2 = 16 u = \pm\sqrt{16} = \pm 4$$

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

$$r =-3 - 4 = -7 s = -3 + 4 = 1$$

Evaluate

$$\left(y-1\right)\left(y+7\right)$$