Mark M. answered 12/02/17
Tutor
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Retired Math prof with teaching and tutoring experience in trig.
m = cscθ - sinθ = 1/sinθ - sinθ = (1-sin2θ)/sinθ = cos2θ/sinθ
Similarly, n = sin2θ / cosθ
So, (m2n)2/3 + (mn2)2/3
= [(cos4θ/sin2θ) (sin2θ/cosθ)]2/3 + [(cos2θ/sinθ)(sin4θ/cos2θ]2/3
= (cos3θ)2/3 + (sin3θ)2/3 = cos2θ + sin2θ = 1