Mark the correct alternative in the following : If in the expansion of , the coefficient of rth and (r +4) th terms are equal, then r is equal to

Question

Philoid · Accepted Answer

Given:

In rth term; k=r-1

& in (r+4)th term ; k=r+3

So, the terms are;

&

Coefficients of both terms are equal:

]

So, r= (21-r);

(r+1)= (20-r);

(r+2)= (19-r);

(r+3)= (18-r)

We get;

r=9