I am taking the GMAT in a couple of weeks. I am doing fine so far, but there are these two annoying questions, that, no matter how hard I try - I do not know how to solve! There are apparently some rules concerning exponents I have missed...
1. 5^21 x 4^11 = 2x 10^n.
The answer is n = 21, but have no idea how to get there.
2. 2^x - 2^(x-2) = 3(2^13)
The answer is x= 15, but again....????
I guess it is uneccessary to say that exponents is not my strongest side :=)
Thanks you for all your help!
Exponential Equations - Solve for X
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Please use the search option before posting a new question . and make sure u post one question per thread...
1 st question
5^21 * 4^11 = 2*10^n
we can write 4^21 as (2^2)^11 which is 2^22
hence its 5^21* 2^22
we can write 2^22 as 2* 2^21 ( a^m+n = a^m* a^n)
hence its
5^21*2*2^21 which can be written as
2* ( 5*2) 21 ...( a^m * b*m = ab^m)
thus its
2* (10)^21 = 2*10^n
hence n = 21
Hope its clear...
1 st question
5^21 * 4^11 = 2*10^n
we can write 4^21 as (2^2)^11 which is 2^22
hence its 5^21* 2^22
we can write 2^22 as 2* 2^21 ( a^m+n = a^m* a^n)
hence its
5^21*2*2^21 which can be written as
2* ( 5*2) 21 ...( a^m * b*m = ab^m)
thus its
2* (10)^21 = 2*10^n
hence n = 21
Hope its clear...
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- Legendary Member
- Posts: 829
- Joined: Mon Jul 07, 2008 10:09 pm
- Location: INDIA
- Thanked: 84 times
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2. 2^x -2^(x-2) = 3(2^13)
u shud know that a^m/a^n = a^m-n...
we have to use the above rule in this question
2^x - 2^x/2^2 = 3( 2^13)........... ( 2^(x-2) can be written as 2^x/2^2 using the rule i mentioned
take 2^x common
2^x ( 1-1/4) = 3 (2^13)
2^x(3/4 ) = 3 (2^13)
cancel off 3 on both the sides..
2^x= 4*2^13
4 can be written as 2^2
hence its
2^x = 2^15 ( a^m* a^n = a^m+n )
hence x = 15...
u shud know that a^m/a^n = a^m-n...
we have to use the above rule in this question
2^x - 2^x/2^2 = 3( 2^13)........... ( 2^(x-2) can be written as 2^x/2^2 using the rule i mentioned
take 2^x common
2^x ( 1-1/4) = 3 (2^13)
2^x(3/4 ) = 3 (2^13)
cancel off 3 on both the sides..
2^x= 4*2^13
4 can be written as 2^2
hence its
2^x = 2^15 ( a^m* a^n = a^m+n )
hence x = 15...