The sum of the squares of the coefficients in the expansion of  (a+b)3 is

(a+b)n=C0   nanb0+C1   nan−1b1+C2   nan−2b2+........+Cn   na0bn ∴(a+b)3=C0   3a3b0+C1   3a2b1+C2   3ab2+C3   3a0b3                  =a3+3a2b+3ab2+b3 The coefficients are 1,3,3,1 Sum  of squares=12+32+32+1                                                                 =1+9+9+1                                                                =20