1983 AHSME Problems/Problem 25: Difference between revisions
| Line 13: | Line 13: | ||
Therefore, we have | Therefore, we have | ||
<cmath>60^{(1-a-b)/2}=4^{1/2}=\boxed{\textbf{(B)}\ 2}</cmath> | <cmath>60^{(1-a-b)/2}=4^{1/2}=\boxed{\textbf{(B)}\ 2}</cmath> | ||
== Solution 2 == | |||
We have <math>60^a = 3</math> and <math>60^b = 5</math>. We can say that <math>a = log_{60} 3</math> and <math>b = log_{60} 5</math>. | |||
<cmath>12^{(1-a-b)/[2(1-b)]} = 12^{(1-(a+b))/[2(1-b)]}</cmath> | |||
We can evaluate (a+b) by the Addition Identity for Logarithms, <math>(a+b) = log_{60} 15</math>. Also, <math>1 = log_{60} 60</math>. | |||
<cmath> (1-(a+b) = log_{60} 60 - log_{60} 15 = log_{60} 4 </cmath> | |||
Now we have to evaluate [2(1-b)], the denominator of the fractional exponent. Once again, we can say <math>1 = log_{60} 60</math> | |||
<cmath> 2(1-b) = 2(log_{60} 12)</cmath> | |||
<cmath>12^{(log_{60} 4)/[2(log_{60} 12]} = 12^{\frac{1}{2} \cdot log_{12} 4} = 4^{1/2} = 2</cmath> | |||
==See Also== | ==See Also== | ||
Revision as of 11:12, 14 January 2022
Problem 25
If 
and 
, then 
is
Solution
We have that 
. We can substitute our value for 5, to get
![\[12=\frac{60}{60^b}=60\cdot 60^{-b}=60^{1-b}.\]](http://latex.artofproblemsolving.com/3/e/9/3e9f343c772c6fcabb7c15c2b8e0ec5e56dfc51b.png)
Hence
![\[12^{(1-a-b)/\left(2\left(1-b\right)\right)}=60^{(1-b)(1-a-b)/\left(2\left(1-b\right)\right)}=60^{(1-a-b)/2}.\]](http://latex.artofproblemsolving.com/6/e/b/6ebb5544f77ce86e1d129a0a929377b0a3a8f9e1.png)
Since 
, we have
![\[4=\frac{60}{60^a 60^b}=60^{1-a-b}.\]](http://latex.artofproblemsolving.com/d/2/f/d2f41a20a20ce8e7f41c6519afb08e70677589f6.png)
Therefore, we have
![\[60^{(1-a-b)/2}=4^{1/2}=\boxed{\textbf{(B)}\ 2}\]](http://latex.artofproblemsolving.com/2/6/9/26995b76464638b54f8741cf13123f3c606016a4.png)
Solution 2
We have 
and 
. We can say that 
and 
.
![\[12^{(1-a-b)/[2(1-b)]} = 12^{(1-(a+b))/[2(1-b)]}\]](http://latex.artofproblemsolving.com/0/7/8/078b538042a748d409b847d511137a3bbb8e0359.png)
We can evaluate (a+b) by the Addition Identity for Logarithms, 
. Also, 
.
![\[(1-(a+b) = log_{60} 60 - log_{60} 15 = log_{60} 4\]](http://latex.artofproblemsolving.com/8/2/6/82605d94d50f8621ba86da396f0bba9ea290d3fa.png)
Now we have to evaluate [2(1-b)], the denominator of the fractional exponent. Once again, we can say
![\[2(1-b) = 2(log_{60} 12)\]](http://latex.artofproblemsolving.com/c/a/d/cadd82875fb52c63f9b0d92432f9c59bf2d325e3.png)
![\[12^{(log_{60} 4)/[2(log_{60} 12]} = 12^{\frac{1}{2} \cdot log_{12} 4} = 4^{1/2} = 2\]](http://latex.artofproblemsolving.com/0/6/e/06e4abe1a5c81a775e3b461e35ee08dc32758346.png)
See Also
| 1983 AHSME (Problems • Answer Key • Resources) | ||
| Preceded by Problem 24 |
Followed by Problem 26 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30 | ||
| All AHSME Problems and Solutions | ||
These problems are copyrighted © by the Mathematical Association of America, as part of the American Mathematics Competitions. Error creating thumbnail: File missing
