Question

if a+b=-(1/3) then proove that a3+b3-ab=-(1/27) solve

Answer 1

(a+b)3=(1/3)3
a3+b3+3a+b(ab)=(1/27)
a3+b3+3*1/3(ab)=1/27
a3+b3+ab=1/27
multiplying by(-1) in both sides
then
a3+b3-ab=-1/27

Answer 2

a+b = -1/3

now cubing both side

(a+b)^3=(-1/3)^3

a^3+b^3+3ab(a+b) = -1/27

now we will put the value of a+b in this which is -1/3 given.

a^3+b^3 +3ab(-1/3) = -1/27

a^3 + b^3 -ab = -1/27

Proved