Question

if x-y = 4 and xy=21 , then find the value of x³- y³​

Answer 1

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Answer 2

Answer:

given:x-y=4.................(1)

xy=21................(2)

x=4+y[from(1)]

putting value of x in equation (2)

(4+y)y=21

4y+y^2=21

y^2+4y-21=0

y^2+7y-3y-21=0

y(y+7)-3(y+7)=0

(y-3)(y+7)=0

putting y-3=0

y=3

Let us take y=3

putting value of y in equation(1)

x-3=4

x=4+3

x=7

now x^3-y^3=

4^3-3^3

=64-9

=55