If the coefficient of second, third and fourth terms in the expansion of (1 + x)2n  are in AP, then 2n2 - 9n is equal to

The general term of (1+x)2n is Tr+1=2nCrxrT2=2nC1x2,T3=2nC2x3,T4=2nC3x4Since , coefficients are in Ap.⇒ 2nC1,2nC2,2nC3 are in AP⇒ 2×2nC2=2nC1+2nC3⇒ 2= 2nC1 2nC2+ 2nC3 2nC2⇒ 2=2(2n−2+1)+2n−3+13⇒ 2=22n−1+2n−23⇒ 2n2−9n+7=0∴ 2n2−9n=−7