The constant term in (1 + x) 10 (1 + 1x) 10 expansion is () A. 1B. (C 110)2C. C120D. C1020

The constant term in (1 + x) 10 (1 + 1x) 10 expansion is () A. 1B. (C 110)2C. C120D. C1020

The general term of (1 + x) 10 (1 + LX) 10 = (1 + x) 20X10 ∵ (1 + x) 20 is tr + 1 = c20rxr. Let r = 10, the coefficient of (1 + x) 20 with X10 term is c2010 ∵ (1 + x) 10 (1 + LX) 10 = (1 + x) 20X10, and the constant term is c2010, so the option is d