If a1, a2, a3 and a4 are the coefficients of four consecutive terms in the expansion of (1+x)n, prove that a1a1+a2+a3a3+a4=2a2a2+a3.
If a1,a2,a3,a4 be the coefficeints of four consecutive terms in the expansion of (1+x)n then prove that a1a1+a2+a3(a3+a4)=2a2(a2+a3).