If the term independent of x in the x−kx210 is 405, then k equals

Tr+1=10Cr(x)10−r−kx2r=10Crx5−5r/2(−k)rFor this to be independent of x,5-5r2=0⇒ r must be 2, so that 10C2k2=405⇒45k2=405⇒k2=9⇒k=±3