Math Is Fun Forum

Lily

Guest

Inverse function of v(t)=10000*0.8t and v-1(4000)

Hi everyone,

I am in trouble finding the inverse function of v(t)=10000*0.8t. I tried but the answer was wrong, could you help me point out where I was wrong. Many thanks!

logv = log(10000*0.8^t)
logv = 4 + tlog0.8
t      = (logv - 4) / log0.8
t       = (log4000 - 4) / log0.8
t       = (log4 + 3 - 4) / (log8 -1)
t       = (log4 -1) / (log8 - 1)

The result is not matched to the reference, which is v^-1(t) = 4.1.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,145

Re: Inverse function of v(t)=10000*0.8t and v-1(4000)

hi Lily,

Welcome to the forum.

Your first step is correct:

Therefore

One way to test if a rearrangement is correct is to choose a value for t and calculate V; then use the new formula to find t when you use that value of V.

Let's try that with t = 1

In my rearranged formula with V = 8000

So that helps to confirm the rearrangement.

The formula you end with,  v^-1(t) = 4.1 gives 8000^-1 = 1/8000 which is not 4.1

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob