Inverse Trigonometric Ratios
Date: 07/01/2003 at 09:27:03 From: Matthew Subject: Inverse sin cos tan etc. Why aren't the inverse trigonometric ratios equal to 1/(the ratio), as is the case for numbers? e.g. sin^-1 doesn't equal 1/sin (or cosec) while x^-1 = 1/x and why isn't then, say, sin^-1 equal to cosec? This is the rule followed for all other numbers but it doesn't apply in this situation involving trigonometric ratios.
Date: 07/01/2003 at 16:07:36 From: Doctor Rob Subject: Re: Inverse sin cos tan etc. Thanks for writing to Ask Dr. Math, Matthew. Yours is an excellent question. The answer involves the fact that the symbol __^(-1) is used for two different purposes, which are confusingly related. If c is a number, then c^(-1) is defined to be the reciprocal of c, or 1/c. If f(x) is a function, then f^(-1) is defined to be the inverse function, that is, the function such that f[f^(-1)(x)] = x. In words, if you apply the inverse function to a value, then apply the function to that result, you get the original value back. In your question, the trigonometric functions are functions, and so the second of these ideas applies. The relation between these is as follows. If c is a nonzero number, there is a function f_c(x) defined by this: for all numbers x, f_c(x) = c*x What is the inverse function of f_c? It turns out that it is defined by this: for all numbers y, (f_c)^(-1)(y) = y/c = (1/c)*y = c^(-1)*y and the inverse function involves the reciprocal of c. Feel free to write again if I can help further. - Doctor Rob, The Math Forum http://mathforum.org/dr.math/
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