What is the coefficient of in the expansion of ?

Answer and explanation

Correct answer: C.

  1. Step 1: Each factor contributes , or . For we need exponents adding to from five factors.
  2. Step 2: Options: four 's and one : ways; two 's, one and two 's: ways; two 's and three 's: ways.
  3. Step 3: The total is .
Answer
.
Why not the others: and miss the five-way case or the ten-way case; and add an extra case that does not give .

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