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WAEC 2022 · Paper 1 · Q25

Find the coefficient of x3y2x^3y^2 in the binomial expansion of (x−2y)5(x - 2y)^5.

Worked solution (try it first)
  1. The term with x3x^3 and y2y^2 is 5C2 x3(−2y)2^{5}C_2\, x^3 (-2y)^2.
  2. 5C2=10^{5}C_2 = 10 and (−2y)2=4y2(-2y)^2 = 4y^2, since squaring removes the minus.
  3. So the term is 40x3y240x^3y^2 and the coefficient is 40, option C.

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