A=P(index0)10^r

If A=80, P(index0)=90, find r

Here's what I did:

A=P(index0)10^r

80=90^r

ln80=ln90^r

r=ln80/ln90

so I get r= 0.973 but actually the solution is supposed to be r=log(8/9) or -0.051153.

What did I do wrong?

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- July 31st 2008, 10:07 AMtami-chan87solving an exponential equation
A=P(index0)10^r

If A=80, P(index0)=90, find r

Here's what I did:

A=P(index0)10^r

80=90^r

ln80=ln90^r

r=ln80/ln90

so I get r= 0.973 but actually the solution is supposed to be r=log(8/9) or -0.051153.

What did I do wrong? - July 31st 2008, 10:30 AMChris L T521
- July 31st 2008, 10:53 AMtami-chan87
Yeah, thanks for your help :)