# Integrating tan(x)

by Omid
Tags: integrating, tanx
 P: 188 Today I was reading my favorite calculus textbook, that saw the integration formula for tan(x). It was : Integral of tan(x) = -ln|cosx| + C . I know that when we say integral of tanx we mean, what is the function whose derivative is tanx. So started to take the derivative of -ln |cosx|, in order to prove the formula. But what could I do with the absolute value sign ? I just ignored it and took the derivative. It worked and I arrived at the answer, tan(x). Now there are 2 questions. 1. why is the sign there anymore? 2. what is the right approach while taking derivative of functions involving absolute value sign? Do we ignore them always, as I did in this case ? Thanks
 Sci Advisor HW Helper P: 1,123 Real values integrate to real values, so if we didn't have the modulus symbol we would be taking the natural logarithm of a negative number when $\cos x < 0$. Anyway you do know to integrate $\tan x$ you just just write it as $$\frac{\sin x}{\cos x}$$?
 Sci Advisor HW Helper PF Gold P: 11,969 The absolute value sign is needed in order to gain the proper integral value of the function $$\frac{1}{x}$$ on intervals where x<0 (Remember, you can't find the natural logarithm of a negative real number among the reals!) To illustrate: Given x>0, we may show that a proper anti-derivative is ln(x). For example, $$\int_{a}^{b}\frac{1}{x}dx=ln(b)-ln(a)=ln(|b|)-ln(|a|)(a,b>0)$$ Let's consider: $$\int_{-b}^{-a}\frac{1}{x}dx$$ Let us make the substitution t=-x: $$\int_{-b}^{-a}\frac{1}{x}dx=\int_{b}^{a}\frac{1}{t}dt=-\int_{a}^{b}\frac{1}{t}dt=-\frac{ln(b)}{ln(a)}$$ Or, further: $$=-\frac{ln(b)}{ln(a)}=\frac{ln(a)}{ln(b)}=ln(|-a|)-ln(|-b|)$$ Hence, we see that a proper anti-derivative valid for both x greater and less than zero is ln|x|

 Related Discussions Calculus 11 Calculus 12 Calculus 12 Calculus & Beyond Homework 10 General Math 9