Find the coefficient of x^7 y^9 in the expansion of (4x+ 5y)^16.
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Expansion of gives us terms which are given by binomial expansion , where ranges from to 0.
Note that powers of and add up to and in the given problem this .
In , we need coefficient of , we have power of and as such
and as such the desired coefficient of is given by
So6 the coefficient is 3.6608E14