Question
Question: Sum of last 20 coefficients in expansion of (1 + x)<sup>39</sup> when expanded in ascending powers o...
Sum of last 20 coefficients in expansion of (1 + x)39 when expanded in ascending powers of x is
A
220
B
238
C
40C20 –219
D
40C20 + 218
Answer
238
Explanation
Solution
S = 39C39 + 39C38 + ........+39C20
39C0+ 39C1 + ....+39C19) + (39C20 +...+39C39) = 239
S + S = 239
Ž S = 238