Expansion frenzy 2

How many rational terms are in the given expansion?

( 3 + 5 4 ) 200 \large (\sqrt{3}+\sqrt[4]{5})^{200}


The answer is 51.

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1 solution

The binomial expansion is ( 3 + 5 4 ) 200 = k = 0 200 ( 200 k ) ( 3 ) 200 k ( 5 4 ) k . (\sqrt 3 + \sqrt[4] 5)^{200} = \sum_{k=0}^{200} \binom {200} k (\sqrt 3)^{200-k} (\sqrt[4] 5)^k. There are 201 terms; however, they are only rational if k k is a multiple of 4 and 200 k 200-k is a multiple of 2. If the former is true, then the latter is also true; thus the rational terms are those with k = 0 , 4 , 8 , , 200 k = 0, 4, 8, \dots, 200 .

There are 51 \boxed{51} of these terms.

They are of the form t 4 n = ( 200 4 n ) 9 50 n 5 n , n = 0 , 1 , 2 , , 50. t_{4n} = \binom{200}{4n}\cdot 9^{50-n}\cdot 5^n,\ \ \ n = 0, 1, 2, \dots, 50.

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