Q.

Let m and n be the coefficients of seventh and thirteenth terms respectively in the expansion of (13x13+12x23)18. Then (nm)13 is:        [2024]

1

19

 

2

14

 

3

49

 

4

94

 


Ans.

(4)

Tr+1=Cr18(13x1/3)18-r·(12x-2/3)r

     Coefficient of 7th term =C618(13)12·(12)6

and coefficient of 13th term =C1218(13)6·(12)12

So, (nm)1/3=[C1218(13)6·(12)12C618(13)12·(12)6]1/3=94