Math help

[yellowstang]

They have an iPhone app for this, but I didn't install it

[red95gts]

y = who gives a fuck?

[Gargamel]

BTW, I fucking hated calculus....

[Osiris]

y= cot^-1 [(dy/dx+2)/cot x]


This isn't with solving the differential equation. It's merely solving for Y.

[TexasDevilDog]

Use sum/difference formula: cos(x-y) = cos(x)cos(y) + sin(x)sin(y)

then it becomes: [[cos(x)cos(y) + sin(x)sin(y)] / sin(x)sin(y)] -1

which simplifies to: dy/dx = cos(x)cos(y)/sin(x)sin(y)

That should get you started.

Thanks. I did not think of using a trig identiy to seperate.

[Nash B.]

Thanks. I did not think of using a trig identiy to seperate.

Nine times out of ten (at least in textbook problems), if there's a trig function in the DE, it's because they want you to use a trig identity. The other time, it's because they want you to waste half an hour looking for a trig identity that doesn't exist and another half hour manipulating the closest thing you could find, before you start all over in a different direction.

[helosailor]

I'm taking remedial algebra, lemme know when you need to break out the big guns.

[jdgregory84]

Oh yeah, why don't you just look in the back of the book for the answer, or is it an even numbered problem?

[TexasDevilDog]

Oh yeah, why don't you just look in the back of the book for the answer, or is it an even numbered problem?

Yes, the back of the book has the answer, but not how it got to it. My TI-89 tells me the answer but doesn't tell me the steps either.

[BERNIE MOSFET]

Thanks. I did not think of using a trig identiy to seperate.

Yeah, trig identities will save you if you learn them inside and out.

I didn't get a chance to finish the problem earlier, but I think if you simplify the dy/dx = cos(x)cos(y)/sin(x)sin(y) further you get dy/dx = cot(x)cot(y)

Which you can separate into tan(y) dy = cot(x) dx

Then integrate and get: ln|sec (y)| = ln|sin(x)| + c

Then take the inverse natural logarithm: sec(y) = sin(x) + e^c

and finally: y = arcsec[sin(x) +e^c]

Is that right?

[Nash B.]

Not quite. Integral of tan x is ln|sin x|, and to cancel the logs, you have to raise e to the power of each side of the equation.

So e^ln|sin y| = e^(ln|cos x| + c), which resolves to
sin y = e^ln|cos x| * e^c = C * cos x

Blah blah blah

To be fair, I Googled the base integrals and trig identities.

[BERNIE MOSFET]

Not quite. Integral of tan x is ln|sin x|, and to cancel the logs, you have to raise e to the power of each side of the equation.

So e^ln|sin y| = e^(ln|cos x| + c), which resolves to
sin y = e^ln|cos x| * e^c = C * cos x

Blah blah blah

Oh, shit, yeah you're right there. That's what I get for doing this on the computer and not checking my work!

Edit: Looked up an integral table again and integral of tan x is ln |sec x| + c, but you got me on the parenthesis.

[Lone Sailor]

Not quite. Integral of tan x is ln|sin x|, and to cancel the logs, you have to raise e to the power of each side of the equation.

So e^ln|sin y| = e^(ln|cos x| + c), which resolves to
sin y = e^ln|cos x| * e^c = C * cos x

Blah blah blah

To be fair, I Googled the base integrals and trig identities.

Cheater.

[Nash B.]

Cheater.

The real world is open book, open notes

[HarrisonTX]

I'm stuck on factoring... That's right, I'm 25 years old and taking high school math. Made the Dean's list though!! LMAO