Question

if x+y=5 then find the value of x^3 + y^3 + 15xy

Answer 1

x+y=5
So, cubing both side
(x+y)^3=5^3
=>x^3 + y^3 +3xy(x+y)=125
=>x^3 + y^3 +15xy=125
as x+y=5

Answer 2

∵x+y=5
∴ cubing both sides,we get,
(x+y)³=5³
=>x³ + y³ +3xy(x+y)=125              {∵(x+y)³=x³+y³+3xy(x+y)}
=>x³+ y³ +15xy=125                     {∵(x+y)=5}
∴x+y=5