Answer:
t = 5,-1,3
Step-by-step explanation:
r=0.01t^3-0.07t^2+0.07t+0.15
For simplification let's multiply the equation by 100
100r = t³ - 7t² + 7t + 15
When r= 0
t³ - 7t² + 7t + 15= 0
Let's look for the value of t.
Let's try a possible division
(t³ - 7t² + 7t + 15)/(t-5) = t² -2t -3
So we've gotten one as t-5
Let's factorize t² -2t -3
= t² +t -3t -3
= t(t+1) -3(t+1)
= (t+1)(t-3)
So we have
(t-5)(t+1)(t-3)
What it means is that the possible values are when
t = 5,-1,3
These are also called the roots of the equation
