Rational terms of expansion

Algebra Level 5

Find the sum of all the rational terms in the expansion of ( 2 1 2 + 3 1 3 + 5 1 6 ) 10 \left( 2^{\frac{1}{2}}+3^{\frac{1}{3}}+5^{\frac{1}{6}} \right)^{10}


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The answer is 11792.

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1 solution

Ahmed Arup Shihab
Feb 27, 2015

After expansion the general term is,

C o n s t a n t × ( 2 1 2 ) x × ( 3 1 3 ) y × ( 5 1 6 ) z Constant \times (2^{\frac{1}{2}})^x \times (3^{\frac{1}{3}})^y \times (5^{\frac{1}{6}} )^z

Where , x + y + z = 10 ( 1 ) x+y+z=10 \dots \dots \dots \dots(1)

A term becomes rational iff x { 0 , 2 , 4 , 6 , 8 , 10 } ; y { 0 , 3 , 6 , 9 } ; z { 0 , 6 } x\in\{0,2,4,6,8,10\} ; \ y\in\{0,3,6,9\} ; \ z\in\{0,6\}

It can be easily found that only ( 10 , 0 , 0 ) , ( 4 , 6 , 0 ) , ( 4 , 0 , 6 ) (10,0,0) , (4,6,0) , (4,0,6) these three triples make ( 1 ) (1) true.

So there are three rational term in the expansion of ( 2 1 2 + 3 1 3 + 5 1 6 ) 10 \left( 2^{\frac{1}{2}}+3^{\frac{1}{3}}+5^{\frac{1}{6}} \right)^{10}

And they are

( 10 10 ) ( 2 1 2 ) 10 × ( 3 1 3 ) 0 × ( 5 1 6 ) 0 = 32 {10\choose10}(2^{\frac{1}{2}})^{10} \times (3^{\frac{1}{3}})^0 \times (5^{\frac{1}{6}} )^0=32

( 10 4 ) ( 2 1 2 ) 4 × ( 3 1 3 ) 6 × ( 5 1 6 ) 0 = 7560 {10\choose4}(2^{\frac{1}{2}})^{4} \times (3^{\frac{1}{3}})^6 \times (5^{\frac{1}{6}} )^0=7560

( 10 4 ) ( 2 1 2 ) 4 × ( 3 1 3 ) 0 × ( 5 1 6 ) 6 = 4200 {10\choose4}(2^{\frac{1}{2}})^{4} \times (3^{\frac{1}{3}})^0 \times (5^{\frac{1}{6}} )^6=4200

So the answer is 32 + 7560 + 4200 = 11792 32+7560+4200=\fbox{11792}

Overrated sum.

Rushikesh Joshi - 6 years, 2 months ago

bhai kya ganda samjhaya

rishi mishra - 4 years, 9 months ago

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