# Another integral

1. Jan 26, 2007

### tony873004

I get the right answer, but...

$$\int_{1/2}^{\sqrt 3 /2} {\frac{6}{{\sqrt {1 - t^2 } }}\,dt}$$

$$F(x) = 6\,\sin ^{ - 1} x$$

$$6\sin ^{ - 1} \frac{{\sqrt 3 }}{2} - 6\sin ^{ - 1} \frac{1}{2} = \pi$$

I only know the answer is pi because I plugged it into my calculator and came out with 6.28... - 3.14... = 3.14...

Is there an easier way besides using the calculator to recognize that this equals pi?

2. Jan 26, 2007

This is a table integral, unless I'm mistaken.

$$\int \frac{dx}{\sqrt{a^2-x^2}}=\arcsin(\frac{x}{a}) + C$$

Edit: this can be shown by using a trig substitution.

3. Jan 26, 2007

### Dick

The exact values of the arcsin function come from working out the angle and side relations in a 30-60-90 triangle (ie half of a equilateral). Maybe you could work them out as a way of reviewing precalc!

4. Jan 26, 2007

### ChaoticLlama

I personally learned integration without anyone making reference to 'tables of integrals'. And I don't see the point in them, you begin to think integration is a matter of memorizing hundreds of anti-derivatives instead of actually learning how to get there. (granted when you do become proficient, use whatever means necessary)

Calculus uses radians :P. you should know that sin(pi / 6) = 1 / 2, and therefore you should be able to figure out what arcsin(1 / 2) is (mind you there are two solutions for all 'arc' functions the exception of critical points).

5. Jan 26, 2007

### Kreizhn

Let $$t=sin(\theta)$$ so that $$dt = cos(\theta) d\theta$$.

Then substitute in, and you should get something like

$$\int\frac{dt}{\sqrt{1-t^2}}$$
$$=\int \frac{cos(\theta)}{cos(\theta)} d\theta$$
$$=\int d\theta$$
$$=\theta + C$$, C the constant of integration
but since $$t=sin(\theta)$$, then $$\theta = arcsin(t)+C$$

This way you won't have to use the table of integrals.

6. Jan 27, 2007

### d_leet

No there aren't, otherwise they wouldn't be functions. The inverses of sines, cosines are defined on restricted domains in order to make them functions.

7. Jan 27, 2007

### HallsofIvy

Staff Emeritus
So the question is NOT about doing the integral, just about finding the exact values of the inverse sine and cosine functions.

You should have already learned some basic values for sine and cosine. As far as $\frac{\sqrt{3}}{2}$ and $\frac{1}{2}$ are concerned, star with an equilateral triangle. Dropping a perpendicular from one vertex to the base gives you two "30-60" right triangles ($\pi/6$ and $\pi/3$ in radians). The hypotenuse of the right triangle is one side of the equilateral triangle, s. The side opposite the $\pi/6$ angle is $\frac{s}{2}$. Use the Pythagorean theorem to determine that the other side, the altitude of the equilateral triangle, is $\frac{s\sqrt{3}}{2}$. From that the sine and cosine values follow:
$$sin(\frac{\pi}{3})= \frac{\sqrt{3}}{2}$$
and
$$sin(\frac{\pi}{6}}= \frac{1}{2}$$

[tex] 6\frac{\pi}{3}- 6\frac{\pi}{6}= 2\pi- \pi= \pi[/itex]

Last edited: Jan 27, 2007
8. Jan 27, 2007