The coefficient of x48 in (1 + x) + 2(1 + x)2 + 3(1 + x)3 + + 100(1 + x)100 is equal to
- 100 100C49 - 100C48
- 100 101C49 - 101C50
- 100C50 + 101C49
- 100 100C49 - 100C50
Answer
(2) 100 101C49 - 101C50
Solution
Let y = 1 + x and S = y + 2y2 + 3y3 + + 100y100 (an A.G.P.)
yS = y2 + 2y3 + + 99y100 + 100y101
Subtracting: S(1 - y) = y + y2 + + y100 - 100y101
S(1 - y) = y101 - yy - 1 - 100y101
1 - y = -x and y - 1 = x
-xS = y101 - yx - 100y101
S = 100(1 + x)101x - (1 + x)101x2 + 1 + xx2
The last term gives only x-2 and x-1.
coefficient of x48 in S
= 100 (coefficient of x49 in (1 + x)101) - (coefficient of x50 in (1 + x)101)
= 100 101C49 - 101C50
Answer: (2)