What are these numbers trying to do?

Algebra Level 3

Challenge: Try to solve this without using logarithm!

If a , b , c , d a, b, c, d and e e are real values that satisfy the system of equations:

2 9 a = 3 1 b = 3 7 d = 4 1 d = 136378 3 23 29^{a} = 31^{b} = 37^{d} = 41^{d} = 1363783^{23}

Find the value of 1 a + 1 b + 1 c + 1 d \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d}

There is exactly one place in solving this problem when you really need a calculator, but not a scientific one. Round your answer to 5 decimal places.


The answer is 0.04348.

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1 solution

1 s t 1^{st} Method : Without Logarithm

2 9 a = 3 1 b = 3 7 d = 4 1 d = 136378 3 23 29^{a} = 31^{b} = 37^{d} = 41^{d} = 1363783^{23}

29 = 136378 3 23 a ( ) 29 = 1363783^{\frac{23}{a}}--------------------(*)

31 = 136378 3 23 b ( ) 31 = 1363783^{\frac{23}{b}}--------------------(**)

37 = 136378 3 23 c ( ) 37 = 1363783^{\frac{23}{c}}--------------------(***)

41 = 136378 3 23 d ( ) 41 = 1363783^{\frac{23}{d}}--------------------(****)

( ) ( ) ( ) ( ) : (*)\cdot(**)\cdot(***)\cdot(****):

1363783 = 136378 3 23 ( 1 a + 1 b + 1 c + 1 d ) 1363783 = 1363783^{23(\frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d})}

1 = 23 ( 1 a + 1 b + 1 c + 1 d ) 1 = 23(\frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d})

1 a + 1 b + 1 c + 1 d = 1 23 0.04348 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} = \frac{1}{23} \approx \boxed{0.04348}

--------------------------------

2 n d 2^{nd} Method : With Logarithm

Let 136378 3 23 = x 1363783^{23} = x

a = l o g 29 x ; 1 a = l o g x 29 ( ) a = log_{29}x; \frac{1}{a} = log_{x}29--------------------(*)

b = l o g 31 x ; 1 b = l o g x 31 ( ) b = log_{31}x; \frac{1}{b} = log_{x}31--------------------(**)

c = l o g 37 x ; 1 c = l o g x 37 ( ) c = log_{37}x; \frac{1}{c} = log_{x}37--------------------(***)

d = l o g 41 x ; 1 a = l o g x 41 ( ) d = log_{41}x; \frac{1}{a} = log_{x}41--------------------(****)

( ) + ( ) + ( ) + ( ) : (*) + (**) + (***) + (****):

1 a + 1 b + 1 c + 1 d = l o g x 29 + l o g x 31 + l o g x 37 + l o g x 41 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} = log_{x}29 + log_{x}31 + log_{x}37 + log_{x}41

1 a + 1 b + 1 c + 1 d = l o g x 1363783 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} = log_{x}1363783

1 a + 1 b + 1 c + 1 d = 1 23 0.04348 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} = \frac{1}{23} \approx \boxed{0.04348}

A great solution!

Saurabh Mallik - 6 years, 9 months ago

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