The sum of the rational terms in the expansion of (2+35)10, is
Let Tr+1 be (r+1)th term in the expansion of
(2+35)10. Then
Tr+1=10Cr(2)10−r(35)r=10Cr×25−r/2×3r/5
Here, 0≤r≤10
For Tr+1 to be rational
r2and r5must be integers.
⇒ r must be a multiple of 10 and 0≤r≤10
⇒ r=0, 10
Thus, T1and T11 are rational terms.
Now,
T1+T11=10C0× 25× 30+ 10C10× 20× 32= 25+ 9=41