How many terms in the expansion of are divisible by
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Any number divisible by 1 2 is divisible by 2 2 × 3 . There are three places where this isn't achieved by direct application of the powers of the coefficients in the expression: the x 2 0 1 4 term, the y 2 0 1 4 term, and the x y 2 0 1 3 term. You might think the answer is 1 2 , but remember that because of the Binomial Theorem, the coefficient of the x y 2 0 1 3 term is ( 2 0 1 3 2 0 1 4 ) × 2 × 3 2 0 1 3 = 4 0 2 8 × 3 2 0 1 3 . This is divisible by 1 2 , so the only two terms that aren't divisible by 1 2 are the x 2 0 1 4 term and the y 2 0 1 4 term. Subtracting this from the 2 0 1 5 terms in the expression, there are 2 0 1 3 terms with coefficients divisible by 1 2 .