Question

If x-y=4 and xy=21 find the value of x 3 - y 3

Answer 1

first you have to find values of x,y after that put into the formula and you will see the answer

Answer 2

Answer:

The value of $x^3-y^3$ is 316.

To find:

The value of $x^3-y^3$

Solution:

Given, $x -y = 4$ and $xy = 21$

$x^3-y^3= ?$

From        

$x- y = 4$

Cubing on both sides,        

$(x- y)^3 = 4^3\\\\x^3- y^3+3.x.(-y)(x-y)= 64\\\\x^3- y^3-3xy(x-y) = 64$

Putting values of (x-y) and xy,  

$x^3- y^3-3.(21)(4) = 64\\\\ x^3- y^3-252 = 64\\\\ x^3- y^3=64+252\\\\x^3-y^3=316$

Hence, the value of $x^3-y^3$ is 316.