If (1+x)n=C0+C1x+C2x2+...+Cnxn,thenC20+C21+C22+.....+C2n is equal to :
2nCn
(1+x)n=C0+C1x+C2x2+....+Cnxn
(x+1)n=C0xn+C1xn−1+C2xn−2+.....+Cn
(1+x)n(x+1)n=(C0+C1x+C2x2+.....+Cnxn)(C0xn+C1xn−1+C2xn−2+.....+Cn)
(1+x)2n=(C0+C1x+C2x2+.....+Cnxn)(C0xn+C1xn−1+C2xn−2+.....+Cn)
Equating the coefficient of xn both sides
C20+C21+C22+.....+C2n=2nCn