If you could solve the equation in the title (and show me how) I'd be very greatful. Thanks.

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- Apr 3rd 2006, 02:29 PMgarybarlow4 sin z + cos z = 0
If you could solve the equation in the title (and show me how) I'd be very greatful. Thanks.

- Apr 3rd 2006, 03:05 PMThePerfectHackerQuote:

Originally Posted by**garybarlow**

Thus,

Thus,

Thus,

Thus,

Thus,

Thus,

Now cheack,

Notice that does not check with the first equation.

However, does.

Thus, you need to solve,

for all - Apr 5th 2006, 11:01 AMmathemagic
4 sin z + cos z = 0

would it not be easier to divide by cos z to get

4 tan z + 1 = 0

and so you'd have to solve tan z = -1/4

that seems easier - Apr 5th 2006, 02:08 PMThePerfectHackerQuote:

Originally Posted by**mathemagic**

- Apr 6th 2006, 01:08 AMmathemagic

because whenever cos z is 0, 4 sin z is either 4 or -4, and so the expression would be -4 + 0 or 4 + 0, neither of which are 0, so you can divide by cos z - Apr 6th 2006, 03:00 AMRebesquesa little trouble!
Well, actually when you are dealing with trigonometric complex functions, a standard trick is to substitute for exponentials. Remember

and after substitution

or

(check this please! calculations are not my strong point :()

so the solution is

or

so that

,

where the logarithm is the usual real one, the principal argument and an integer. - Apr 6th 2006, 03:47 AMmathemagic
I've not learnt about the exponential stuff you can do with trigonometric equations like that yet.

I thought of another way to solve it

4 sin z + cos z = 0

let:

4 sin z + cos z = k cos(z - a)

4 sin z + cos z = k cos a cos z + k sin a sin z

k sin a = 4

k cos a = 1

=> tan a = 4

a = 1.33

4 sin z + cos z = k cos(z - 1.33)

k = sqrt(4^2 + 1^2)

k = sqrt(17)

4 sin z + cos z = sqrt(5) cos(z - 1.33)

so now you need to solve

sqrt(17) cos(z - 1.33) = 0 - Apr 6th 2006, 04:01 AMRebesquesQuote:

cos(z - 1.33) = 0

- Apr 6th 2006, 04:10 AMmathemagic
to solve sqrt(17)cos (z - 1.33) = 0

you do this:

z- 1.33 = cos -1 (0)

= pi/2, 3pi/2 ...

z = pi/2 + 1.33, 3pi/2 + 1.33 etc - Apr 6th 2006, 04:50 AMRebesques
Oh, :confused: I just realised what :eek: Seems ok!

- Apr 6th 2006, 06:59 AMThePerfectHacker
I think we should keep the math simple for the high school students. Not everyone here knows advanced math.