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 04-18-10, 07:26 AM #1 Yan  BeaverTerror Thread Starter     Join Date: Jan 2006 Location: Shanghai, China Bikes: 1995 Kestrel 4000; 2013 True North Touring; 1989 Miele Tivoli; 1979 Colnago Sport Posts: 1,947 Mentioned: 3 Post(s) Tagged: 0 Thread(s) Quoted: 176 Post(s) 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. __________________ Yan Last edited by Yan; 04-18-10 at 07:32 AM.
04-18-10, 09:25 AM   #2
Siu Blue Wind
I don't think so

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f) 42
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04-18-10, 09:47 AM   #3
jsharr
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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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 04-18-10, 10:56 AM #4 phantomcow2 la vache fantôme     Join Date: Aug 2004 Location: NH Bikes: Posts: 6,266 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) 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. __________________ C://dos C://dos.run run.dos.run
 04-18-10, 10:56 AM #5 phantomcow2 la vache fantôme     Join Date: Aug 2004 Location: NH Bikes: Posts: 6,266 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) Oh, didn't see your edit. Glad you solved it. __________________ C://dos C://dos.run run.dos.run
04-18-10, 12:46 PM   #6
Dan The Man
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Quote:
 Originally Posted by Yan 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.

 04-18-10, 01:11 PM #7 Snicklefritz Senior Member     Join Date: Mar 2005 Location: In the middle of horse country, in The Garden State Bikes: Posts: 3,159 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Where did you find this question?
 04-18-10, 07:15 PM #8 Gilla Gorilla 先輩     Join Date: Aug 2009 Location: In a desert somewhere Bikes: A black P0S (piece of steel) =P Posts: 71 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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....
04-18-10, 08:39 PM   #9
recon455
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 Originally Posted by gilla gorilla here's another question for you: Whats the integral of 2xdx between 10 and 13? =p
69.
Ha
ha
ha.

04-18-10, 08:47 PM   #10
phantomcow2
la vache fantôme

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 Originally Posted by recon455 69. Ha ha ha.
no credit without work:
F(x) = x^2
F(b) - F(a) = 169-100=69
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 04-18-10, 10:38 PM #11 alexvpaq Senior Member     Join Date: May 2009 Location: Qc,Canada Bikes: A trek Posts: 991 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) I prefer chemistry class GO PV=nRT!
04-18-10, 10:42 PM   #12
UmneyDurak
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Quote:
 Originally Posted by Gilla Gorilla here's another question for you: Whats the integral of 2xdx between 10 and 13? =P
I used to remember how to do that.

 04-20-10, 04:40 PM #13 fantom1  Senior Member   Join Date: Mar 2004 Location: Middle of the ocean Bikes: Posts: 400 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 52 Post(s) math is the devil.
 04-20-10, 05:19 PM #14 deraltekluge Senior Member     Join Date: Sep 2006 Bikes: Kona Cinder Cone, Sun EZ-3 AX Posts: 1,195 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Young man, in mathematics you don't understand things. You just get used to them. ~John von Neumann~