Q1 of 51 Page 18

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


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

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