Question

if interest by equals to 5 then find the value of x cube + y cube + 125 + 15 x y

Answer 1

Just substitute the value of X & Y according to given data, i.e (x+y) =5

so the possible value to substitute isX=0

X=0 , Y=5

X=1 , Y=4

X=2 , Y=3

X=3 , Y=2

X=4 , Y=1

X=5 , Y=0

put any of the 2 corresponding values and you will find the result as 0 for every case.

There is an alternate method as well:

Given that x+y=5

cube both sides:

(x+y)^3=x^3+y^3+3xy(x+y)

125=x^3+y^3+3xy(5)

hence

x^3+y^3+15xy=125

so the required expression is x^3+y^3+15xy-125=125–125=0

Hope This Helps :)