We know that tanx = sinx/ cosx and secx =1/cosx. And the trigonometric identity, (sinx)^2+(cosx)^2 = 1
Left side:
cosx(tanx)^2 +cosx = cosx(sinx/cosx)^2+cosx = (sinx)^2/cosx+cosx = (sinx)^2 / cosx + cos x*cosx/cosx = [(sinx)^2+(cosx)^2]/cosx = 1/cosx =secx which is the term on RHS.
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