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We tried to do an integral with the universal trigonometric substitution $$\int \frac{1}{(1+\sin x)}dx$$

Meaning, we substituted: $ t = \tan \frac{x}{2} \Rightarrow$

$$\int \frac{1}{(1+\sin x)}dx = \int \frac{\frac{2}{1+t^2}}{1+\frac{2t}{1+t^2}}dt = \int \frac{2}{(1+t)^2}dt = \frac{-2}{1+t} = \frac{-2}{1+\tan \frac{x}{2}} + C$$

But the answer is: $$ \tan x - \frac{1}{\cos x} + C $$

What did we do wrong?

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I think your approach is correct. Manipulate a little bit your final result and you will come up with the answer. –  Dmoreno Jan 21 at 10:44
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3 Answers 3

up vote 14 down vote accepted

$$\begin{align} \frac{-2}{1+\tan \frac{x}{2}} &= \frac{-2\cos \frac{x}{2}}{\cos\frac{x}{2} + \sin \frac{x}{2}}\\ &= \frac{-2\cos \frac{x}{2}(\cos \frac{x}{2}-\sin\frac{x}{2})}{(\cos \frac{x}{2}+\sin\frac{x}{2})(\cos \frac{x}{2}-\sin\frac{x}{2})}\\ &= \frac{\sin x - 2 \cos^2 \frac{x}{2}}{\cos x}\\ &= \tan x - \frac{\cos x + 1}{\cos x}\\ &= \tan x - \frac{1}{\cos x} - 1 \end{align}$$

You did nothing wrong, you just got another representation of the same family of functions.

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when I put the functions in WolframAlpha, it say's that the functions are not equal for every x... –  Tomer Jan 21 at 10:44
    
You have different constants. Plug in $x = 0$ to see that the main part of your primitive and the answer's is $1$ there. Differentiate to see that both are primitives of the integrand, so the difference is constant. –  Daniel Fischer Jan 21 at 10:47
    
ok, thank you!! –  Tomer Jan 21 at 10:49
    
@Tomer, I'm no math whiz, so I can't directly comment on the correctness of Daniel's answer. However, Wolfram Alpha, when not given any constraints, takes the broadest possible approach to the problem. Wolfram will consider possibilities such as imaginary values for x, even if you think "x is a real number" is implied. This sort of thing is, in my experience, the most common logical error when dealing with Wolfram technologies. –  Brian S Jan 21 at 19:20
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An alternative method of integration first involves multiplying the numerator and denominator by $1-\sin x$: this gives $$\begin{align*} \int \frac{1}{1+\sin x} \, dx &= \int \frac{1 - \sin x}{1 - \sin^2 x} \, dx \\ &= \int \frac{1 - \sin x}{\cos^2 x} \, dx \\ &= \int \sec^2 x - \frac{\sin x}{\cos^2 x} \, dx \\ &= \tan x - \int \frac{-du}{u^2}, \quad u = \cos x, du = -\sin x \, dx \\ &= \tan x - \frac{1}{u} + C \\ &= \tan x - \sec x + C. \end{align*}$$ To see the equivalence of this form with the expression $$-\frac{2}{1+\tan \frac{x}{2}} + C,$$ consider their difference, with $\theta = x/2$: $$\begin{align*} \tan x - \sec x + \frac{2}{1 + \tan \frac{x}{2}} &= \frac{\sin 2\theta - 1}{\cos 2\theta} + \frac{2}{1 + \tan \theta} \\ &= \frac{\sin 2\theta - 1}{\cos 2\theta} + \frac{2 \cos \theta}{\sin \theta + \cos \theta} \\ &= \frac{\sin 2\theta - 1 + 2 \cos\theta(\cos \theta - \sin \theta)}{\cos 2\theta} \\ &= \frac{2 \cos^2 \theta - 1}{\cos 2\theta} \\ &= 1. \end{align*}$$ Thus their difference is constant, and both are antiderivatives.

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Another approach would be to first multiply the integrand by $\dfrac{1-\sin x}{1-\sin x}$ to get $$\begin{aligned}\int \frac{1}{1+\sin x}\,dx &= \int\frac{1-\sin x}{1-\sin^2 x}\,dx\\ &= \int\frac{1-\sin x}{\cos^2x}\,dx \\ &= \int\frac{1}{\cos^2 x}-\frac{\sin x}{\cos^2 x}\,dx \\ &= \int \sec^2x -\frac{\sin x}{\cos^2x}\,dx\\ &= \ldots\end{aligned}$$ The first term is trivial to integrate, and the second integral requires the substitution $u=\cos x$. I leave it to you to finish things off.

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