Question

Find the value of x^3-y^3-9xy , if (x-y)=4

Answer 1

Simply cube the equation

So (x−y)³=x³−y³−3xy(x−y)=4³=64

But we know x−y=4 So substituting in the equation

So x³−y³−3xy(3)=x³−y³−9xy=64