Mathani H. answered 06/22/16
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note: tan(x)=sin(x)/cos(x) and sec(x)= 1/cos(x)
then on the right side:
tan(x)sin(x) + cos(x) =[ (sin(x)) / (cos(x)) x sin(x) ] + cos(x)
[sin^2(x) + cos^2(x) / cos(x)] = 1/cos(x) = left side = sec(x)