Question

find the value of x cube + y cube + 15 x y - 125 if X + Y is equals to 5​

Answer 1

x+y=5

cubing on both sides

(x+y)^3=125

x^3+y^3+3xy(x+y)=125

x^3+y^3+3xy(5)=125. (Since x+y=5)

x^3+y^3+15xy=125

Thus

x^3+y^3+15xy-125=0