First slide
Binomial theorem for positive integral Index
Question

The sum of rational terms in the expansion of  (2+31/5)10 is

Moderate
Solution

(r + 1)th term in the given expansion is given by tr+1=10Cr 210r2 3r5
where r = 0, 1, 2, ..., 10
For rational terms
r = a multiple of 5 = 0, 5, 10……..(1)
10 – r = a multiple of 2 = 0, 2, 4, 6, 8, 10 ……...(2)
From Eq. (1) and (2) possible values of r are : 0 and 10
∴ sum of rational terms

 =t1+t11=10C0(2)10(31/5)0+10C10(2)0(31/5)10 =25+32=32+9=41

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