The sum of the series 2 1 20C4 - 3 2 20C5 + 4 3 20C6 - 5 4 20C7 + + 18 17 20C20, is equal to .
Answer
34
Solution
The general term (r = 4 to 20) is (-1)r (r - 2)(r - 3) 20Cr.
(r = 4 gives +2 1 20C4, r = 20 gives +18 17 20C20.)
Since (r - 2)(r - 3) is a polynomial in r of degree 2 (< 20),
r=020 (-1)r (r - 2)(r - 3) 20Cr = 0
(because (-1)r nCr = (-1)r r nCr = (-1)r r(r - 1) nCr = 0 for n = 20)
S = - r=03 (-1)r (r - 2)(r - 3) 20Cr
r = 0: (+1)(-2)(-3)(1) = 6
r = 1: (-1)(-1)(-2)(20) = -40
r = 2, 3: the factor (r - 2)(r - 3) = 0
S = -(6 - 40) = 34
Answer: 34