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p+q=-8 pq=1\times 15=15
Factor the expression by grouping. First, the expression needs to be rewritten as b^{2}+pb+qb+15. To find p and q, set up a system to be solved.
-1,-15 -3,-5
Since pq is positive, p and q have the same sign. Since p+q is negative, p and q are both negative. List all such integer pairs that give product 15.
-1-15=-16 -3-5=-8
Calculate the sum for each pair.
p=-5 q=-3
The solution is the pair that gives sum -8.
\left(b^{2}-5b\right)+\left(-3b+15\right)
Rewrite b^{2}-8b+15 as \left(b^{2}-5b\right)+\left(-3b+15\right).
b\left(b-5\right)-3\left(b-5\right)
Factor out b in the first and -3 in the second group.
\left(b-5\right)\left(b-3\right)
Factor out common term b-5 by using distributive property.
b^{2}-8b+15=0
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.
b=\frac{-\left(-8\right)±\sqrt{\left(-8\right)^{2}-4\times 15}}{2}
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.
b=\frac{-\left(-8\right)±\sqrt{64-4\times 15}}{2}
Square -8.
b=\frac{-\left(-8\right)±\sqrt{64-60}}{2}
Multiply -4 times 15.
b=\frac{-\left(-8\right)±\sqrt{4}}{2}
Add 64 to -60.
b=\frac{-\left(-8\right)±2}{2}
Take the square root of 4.
b=\frac{8±2}{2}
The opposite of -8 is 8.
b=\frac{10}{2}
Now solve the equation b=\frac{8±2}{2} when ± is plus. Add 8 to 2.
b=5
Divide 10 by 2.
b=\frac{6}{2}
Now solve the equation b=\frac{8±2}{2} when ± is minus. Subtract 2 from 8.
b=3
Divide 6 by 2.
b^{2}-8b+15=\left(b-5\right)\left(b-3\right)
Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 5 for x_{1} and 3 for x_{2}.
x ^ 2 -8x +15 = 0
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.
r + s = 8 rs = 15
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 = 4 - u s = 4 + u
Two numbers r and s sum up to 8 exactly when the average of the two numbers is \frac{1}{2}*8 = 4. 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>
(4 - u) (4 + u) = 15
To solve for unknown quantity u, substitute these in the product equation rs = 15
16 - u^2 = 15
Simplify by expanding (a -b) (a + b) = a^2 – b^2
-u^2 = 15-16 = -1
Simplify the expression by subtracting 16 on both sides
u^2 = 1 u = \pm\sqrt{1} = \pm 1
Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u
r =4 - 1 = 3 s = 4 + 1 = 5
The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.