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Need integral help

Started by griffenjam Sep 10, 2003 at 1:37 PM 6 replies 750+ views

Original Post

Sep 10, 2003 at 1:37 PM

I seem to be out of pratice with my integrals and need some help. I have two that I was unable to do. I know I could have done these just 6 months ago, but I seem to have a mental block right now. Could I get some help with (Sigma) 1/(cos(x) -1) dx and (sigma) sin(5x)*sin(2x) dx On the second one I was thinking of subbing with u=2x, du=2 dx then having 1/2 (sigma) sin((5/2)u)*sin(u) du but that doesn''t help much. Or changing to something like this (sigma) sin(3x + 2x)*2sin(x)cos(x) dx but that it still a lot of work. I get the feeling that I am overlooking something

ShlomiSteinberg

Sep 10, 2003 at 1:40 PM

NOOOO!!! I hate integrals!!

"C lets you shoot yourself in the foot rather easily. C++ allows you to reuse the bullet!"

lnxwndusr

Sep 10, 2003 at 7:43 PM

quote:
Original post by griffenjam

(Sigma) 1/(cos(x) -1) dx

I assume you mean ∫ not Σ?

∫ dx/(cos(x)-1) = cot(x/2), methinks.

Not sure 'bout num 2 though.

[edited by - lnxwndusr on September 10, 2003 8:44:58 PM]

Senses777

Sep 10, 2003 at 7:48 PM

The is the purpous of integrals?

Far too much time has passed
For us to lament that we were in love.

Taoist

Sep 10, 2003 at 8:03 PM

It''s a reverse derivative. Or "area under the curve".

"The softest substance of the world
Goes through the hardest.
That-which-is-without-form penetrates that-which-has-no-crevice;
Through this I know the benefit of taking no action.
The teaching without words
And the benefit of taking no action
Are without compare in the universe." - Book of Tao

Taoist aka Sailorstick

Colin Jeanne

Sep 10, 2003 at 8:29 PM

quote:
Original post by griffenjam
(sigma) sin(5x)*sin(2x) dx

Or changing to something like this
(sigma) sin(3x + 2x)*2sin(x)cos(x) dx
but that it still a lot of work.

Continue by expanding the sin(3x + 2x) until you are left with sin(x)s, cos(x)s, and constants. That might get you somewhere.

Colin Jeanne | Invader''s Realm

Cold_Steel

Sep 10, 2003 at 8:42 PM

May I suggest a good table of integrals? If you don''t actually need to show your work, go with those. Or if the work involved will take too long. I used to be really good at calculus, but I don''t use it very often, so my ability sort of went away.

Beer Hunter

Sep 10, 2003 at 9:16 PM

I recommend integrals.com — it''s very useful.

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