SOLUTION: Find all zeros of f(x)=3x^3+9x^2-5x-15

Algebra ->  Equations -> SOLUTION: Find all zeros of f(x)=3x^3+9x^2-5x-15      Log On


   

Question 696969: Find all zeros of f(x)=3x^3+9x^2-5x-15
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!

f%28x%29=3x%5E3%2B9x%5E2-5x-15...set f%28x%29 equal to zero

3x%5E3%2B9x%5E2-5x-15=0...factor
%283x%5E3-5x%29%2B%289x%5E2-15%29=0
x%283x%5E2-5%29%2B3%283x%5E2-5%29=0
%28x%2B3%29%283x%5E2-5%29=0
solutions:
if x%2B3=0....=>...highlight%28x=-3%29
if 3x%5E2-5=0...=>...3x%5E2=5..=>...x%5E2=5%2F3..=>...highlight%28x=sqrt%285%2F3%29%29 and highlight%28x=-sqrt%285%2F3%29%29
or sqrt%285%2F3%29=1.29, so we have
highlight%28x=1.29%29 and highlight%28x=-1.29%29