Question

Coefficient of y^15x^2 in expansion of (y^3+x)^7

Answer 1

The coefficient of y∧15×∧2 in expansion of (y∧3+x)∧7 is (15×2)+(3×7)= 51

Answer 2

Hello,


$(x+y^{3})^7 =$
$x^7+7x^{6}y^{3}+21x^{5}y^{6}+35x^{4}y^{9}+35x^{3}y^{12}+21x^{2}y^{15}+7xy^{18}+y^{21}$

Coefficient=21=2P7=7*6/2!