Question

If x—y=4and x y=21, then x³ —y³

Answer 1

x³-y³= (x-y) (x²+xy+y²)
= 4(x²+y²+21)..... eq 1
y= 21/x
x-21/x=4
x² - 21 - 4x = 0
x= - 7 or x= 4
Here, x= - 7 is valid. y= -3
Eq 1,
Req. ans: = 4( 49+9+21)
=316