e的h次方=secx 為什等於 e的-h次方=cosx?

e^h=sec x
h=ln sec x
-h=-ln sec x
-h=-ln (1/cos x)
-h=-(ln 1 - ln cos x)
-h=-(0 - ln cos x)
-h=ln cos x
e^-h=cos x

因為 sec x = 1 / cos x = [ cos x ]⁻¹

e^h = sec x = [ cos x ]⁻¹
則 e^-h = cos x

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