# Homework Help: Simple, yet ambiguous integral question

1. Mar 19, 2016

### TheExibo

1. The problem statement, all variables and given/known data

Evaluate the definite (from -0.3 to 0.3) integral ∫tanxdx

2. Relevant equations

(dy/dx)tanx=(secx)^2

3. The attempt at a solution

Using the anti-derivative, I got to (sec(0.3))^2-(sec(-0.3))^2, which gives approximately 2. However, if the derivative is sketched out, and the area underneath is found, the answer is then 0.

I found this happening with other similar questions as well. What could be going on?

2. Mar 19, 2016

### ShayanJ

$\frac{d}{dx} \tan x=\sec^2 x$ implies $\int \sec^2 x \ dx=\tan x$, not $\int \tan x \ dx=\sec^2 x$!
$\tan x$ is an odd function of x so its definite integral over -b to b, for any real b, is 0.
Of course, for some b, the interval may contain a singular point of $\tan x$ in which case the definite integral would diverge.

3. Mar 19, 2016

### Ray Vickson

You have the wrong antiderivative, but that is not your only error: you should get an answer of 0 (not $\approx$ 2) because $\sec^2(0.3) = \sec^2(-0.3)$.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted