SOLUTION: Really need some help please. Given the quadratic function f(x) = x2 – 18x + 81, find a value of x such that f(x) = 16.

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: Really need some help please. Given the quadratic function f(x) = x2 – 18x + 81, find a value of x such that f(x) = 16.      Log On


   

Question 493725: Really need some help please.
Given the quadratic function f(x) = x2 – 18x + 81, find a value of x such that f(x) = 16.
Found 2 solutions by nerdybill, MRperkins:
Answer by nerdybill(7384) About Me  (Show Source):
You can put this solution on YOUR website!
Given the quadratic function f(x) = x2 – 18x + 81, find a value of x such that f(x) = 16
.
substitute in the value of f(x):
f(x) = x^2 – 18x + 81
16 = x^2 – 18x + 81
0 = x^2 – 18x + 65
factoring:
0 = (x-5)(x-13)
x = {5, 13} (two possible solutions)

Answer by MRperkins(300) About Me  (Show Source):
You can put this solution on YOUR website!
let f(x)=16, and substitute it into the equation
16=x%5E2-18x%2B81
0=x%5E2-18x%2B65
factor and get %28x-13%29%28x-5%29=0
zero product rule says that one of them, (x-13) or (x-5) must be 0 for this to be true, so set each DISABLED_event_one= to zero and solve
x=13, x=5
x=5,13
You can then check these by substituting them back into the original equation.