Answer
See below.
Work Step by Step
$RHS=\frac{sin^3\theta+cos^3\theta}{sin\theta+cos\theta}
=\frac{(sin\theta+cos\theta)(sin^2\theta+cos^2\theta-sin\theta cos\theta)}{sin\theta+cos\theta}
=1-sin\theta cos\theta=1-\frac{1}{2}sin(2\theta)=LHS$
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