First slide
Binomial theorem for positive integral Index
Question

The sum of the rational terms in the expansion of (2+35)10, is

Moderate
Solution

Let Tr+1 be (r+1)th term in the expansion of

(2+35)10. Then

Tr+1=10Cr(2)10r(35)r=10Cr×25r/2×3r/5

Here, 0r10

For Tr+1 to be rational

r2and r5must be integers. 

 r must be a multiple of 10 and 0r10

 r=0, 10

Thus, T1and T11 are rational terms. 

Now,

T1+T11=10C0× 25× 30+ 10C10× 20× 32= 25+ 9=41

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