9514 1404 393
Answer:
(a) -29,884,416
(b) C(n,(n -k)/2) . . . where C(n,k) = n!/(k!(n-k)!) and k ∈ ℕ
Explanation:
(a) The coefficient of the k-th term of the expansion is ...
C(20,k)(3x)^(20-k)(-2y)^k
For k=19, this is ...
19(3x)(-2y)^19 = -29,884,416xy^19
The coefficient of the term is -29,884,416.
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(b) The p-th term of the expansion of (x +1/x)^n is ...
C(n,p)(x^(n-p)(1/x)^p = C(n,p)(x^(n-2p))
For some exponent k of x, the value of p will be ...
k = n -2p
p = (n -k)/2
Then the coefficient of x^k will be C(n, (n-k)/2), where C(n, x) = n!/(x!(n-x)!).
You will notice that there will only be a term x^k for k having the same parity as n.