# Integration By Trig Substitution

1. Aug 31, 2010

### neshepard

1. The problem statement, all variables and given/known data
∫√(4-x^2)/x dx

2. Relevant equations

3. The attempt at a solution
a^2=4 u^2=x^2 ⇒ u=asinθ
a=2 u=x
x=2sinθ sinθ
2cosθ=√(4-x^2)
dx=2cosθ dθ

∫√(4-4sin^2θ)/2sinθ 2cosθ dθ
∫2cos^2θ/2sinθ 2cosθ dθ
2∫cos^2θ/sinθ cosθ dθ

Now what????????????? I have been working on this problem for the last 3 3 hours and I am at a stand still. Can somebody Please get me to the answer?????????

2. Aug 31, 2010

### rock.freak667

Now use the formula cos2θ+sin2θ=1 and replace cos2θ and then try using t=sinθ as a substitution.

3. Aug 31, 2010

### Dick

I don't think you want to use a trig substitution. Try u^2=4-x^2. Then use partial fractions on the u integral.

4. Aug 31, 2010

### neshepard

To do partial fractions, don't I need to divide the numerator by the denominator because the exponent is larger in the numerator? If so, how? I've never tried to do synthetic division on a sqrt.

5. Aug 31, 2010

### Dick

Maybe you missed that it's u^2=4-x^2, not u=4-x^2. There's no sqrt left after the u substitution.

6. Aug 31, 2010

### neshepard

What I have now is:
=2∫cos^2θ/sinθ dθ
=2∫1-sin^2θ/sinθ dθ
=2∫1-sinθ dθ
=2(cosθ + θ)
But I can't resub because there is no value for θ!

7. Sep 1, 2010

### Dick

You are also making algebra mistakes. (1-sin(t)^2)/sin(t) isn't 1-sin(t). And to get things back in terms of x, you want to put theta=arcsin(x/2).

8. Sep 1, 2010

### sachinism

well thats right

whenever u have anything under the square root , try removing hte square root first