1 + tan²x = sec²x (1 + tan²x = sec²x)

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1 + tan²x = sec²x

What each symbol means

SymbolWhat it stands for
xAngle where cos x ≠ 0 (x ≠ odd multiple of π/2)

When to use this

x ≠ (2n+1)π/2. Derived by dividing sin²x+cos²x=1 by cos²x.