Note: In such types of question always assume a complex number in the form of$\left( {z = \cos \theta + i\sin \theta } \right)$, then remember the standard known fact which is if $a + b + c = 0$, then ${a^3} + {b^3} + {c^3} = 3abc$, then simplify according to given conditions then compare real and imaginary parts we will get the required answer.
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