That is simply the trig identity
sin^2 x + cos^2 x = 1
You can show it easily with a right triangle with 90 deg at C where
a^2 + b^2 = c^2
sin A = a/c
cos A = b/c
so
sin^2 A = a^2/c^2
cos^2 A = b^2/c^2
and we know
c^2 sin^2 A + c^2 cos^2 A = c^2
so
sin^2 A + cos^2 A = 1
Sin^2x-1=-cos^2x
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