- #1

- 665

- 26

I believe whether they are positive or negative is by a choice of convention. So why don't we define them to be both positive or both negative?

##\cos^{-1}x## graph: http://www.wolframalpha.com/input/?i=y=acos(x)

##\cosh^{-1}x## graph: http://www.wolframalpha.com/input/?i=y=acosh(x)

Question 2:

Since the real part of ##\cosh^{-1}x## is the imaginary part of ##\cos^{-1}x## when ##x>1##, this means that multiplying ##i## to the real part of ##\cosh^{-1}x## gives the imaginary part of ##\cos^{-1}x##:

##i\cosh^{-1}x=\cos^{-1}x## for ##x>1##.

Similarly, since the real part of ##\cos^{-1}x## is the imaginary part of ##\cosh^{-1}x## when ##x<1##, we have

##\cosh^{-1}x=i\cos^{-1}x## for ##x<1##.

We have two rules depending on the value of ##x##. Why not use the same rule? And change the definition of ##\cosh^{-1}x## accordingly: define the real part of ##\cosh^{-1}x## as negative for ##x>1##.