TRY IT!!!!

hey m not getting the solution!
2k=???
how?

plz help!

the ques is not clearly written

can u be more clear??

@rajatjain;

they have just used the 2 expressions for k given above, combined them and then manipulated to get the answer...

forget about the sigma marks and its limits, see inside as justa n entity, and work out

u will get the answer