Begin with simplifying the problem by taking the natural log of both sides of the equation.
sin2(x) + tan2(x) = sec2(x) - cos2(x)
Use the identity tan2(x) + 1 = sec2(x)
sin2(x) + sec2(x) - 1 = sec2(x) - cos2(x)
sin2(x) - 1 = -cos2(x)
sin2(x) + cos2(x) = 1
confirmed