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Old 04-18-10, 07:26 AM   #1
Yan 
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Can you solve this math problem?

I can't figure this out and I feel stupid. Help me out here:

Multiple choice, no calculator allowed:

If [2^x]-[2^(x-2)]=3(2^13), what is the value of x?

a) 9
b) 11
c) 13
d) 15
e) 17

The answer is d, but how do you solve this without a calculator?

edit: Nevermind, got it.
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Last edited by Yan; 04-18-10 at 07:32 AM.
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Old 04-18-10, 09:25 AM   #2
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f) 42
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Old 04-18-10, 09:47 AM   #3
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I try not to put a value on x. Last time I did, he thought it was a bounty and he turned himself into me. I thought he would never leave, and it cost $75 bucks.
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Old 04-18-10, 10:56 AM   #4
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Usually when your variable (x) is a in exponent form, you take the natural log of both sides. Recall (hopefully) the handy power of logs:

If you take the log of a^x, where a is a constant, the x goes in front. This results in x(ln(a)). Take the logs of both sides and you'll probably find ln(2) cancels or something.
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Old 04-18-10, 10:56 AM   #5
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Oh, didn't see your edit. Glad you solved it.
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Old 04-18-10, 12:46 PM   #6
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Originally Posted by Yan View Post
I can't figure this out and I feel stupid. Help me out here:

Multiple choice, no calculator allowed:

If [2^x]-[2^(x-2)]=3(2^13), what is the value of x?

a) 9
b) 11
c) 13
d) 15
e) 17

The answer is d, but how do you solve this without a calculator?

edit: Nevermind, got it.
3(2^13) is (0.75*2*2)*2^13. That is 0.75*2^15. Easy stuff. 2^(x-2) is going to be 1/4 the value of 2^x right. So 3/4 of 2^x equals 3/4 of 2^15. Guess what x equals.
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Old 04-18-10, 01:11 PM   #7
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Where did you find this question?
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Old 04-18-10, 07:15 PM   #8
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here's another question for you: Whats the integral of 2xdx between 10 and 13?
=P

Last edited by Gilla Gorilla; 04-18-10 at 07:16 PM. Reason: i guess this forum doesn't like calculus....
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Old 04-18-10, 08:39 PM   #9
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here's another question for you: Whats the integral of 2xdx between 10 and 13?
=p
69.
Ha
ha
ha.
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Old 04-18-10, 08:47 PM   #10
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69.
Ha
ha
ha.
no credit without work:
F(x) = x^2
F(b) - F(a) = 169-100=69
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Old 04-18-10, 10:38 PM   #11
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I prefer chemistry class GO PV=nRT!
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Old 04-18-10, 10:42 PM   #12
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here's another question for you: Whats the integral of 2xdx between 10 and 13?
=P
I used to remember how to do that.
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Old 04-20-10, 04:40 PM   #13
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math is the devil.
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Old 04-20-10, 05:19 PM   #14
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Young man, in mathematics you don't understand things. You just get used to them.
~John von Neumann~
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