Question

If x+y−7=0, find the value of x^3+y^3+21xy−343.

Answer 1

Solution

x+y-7=0

x+y=7

(cubing both sides)

⇒(x+y)³=(7)³

⇒x³+y³+3(x+y)xy=343

⇒x³+y³+21xy=343