solve: cos(5x) + sin(x) = 0

I have that:

(sin(3x) + cos(3x))(cos(2x) - sin(2x)) = 0

therefore:

sin(3x) = -cos(3x) and cos(2x) = sin(2x)

from here I'm stuck... any help would be much appreciated :)

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- January 8th 2013, 09:59 AMkinhew93Trigonometric equation
solve: cos(5x) + sin(x) = 0

I have that:

(sin(3x) + cos(3x))(cos(2x) - sin(2x)) = 0

therefore:

sin(3x) = -cos(3x) and cos(2x) = sin(2x)

from here I'm stuck... any help would be much appreciated :) - January 8th 2013, 10:36 AMMarkFLRe: Trigonometric equation
Let's look at the first factor and consider where:

Divide through by and so obviously :

where .

So, what is in this case?

Can you use a similar method for the first factor? - January 8th 2013, 11:05 AMkinhew93Re: Trigonometric equation
- January 8th 2013, 11:17 AMMarkFLRe: Trigonometric equation
Yes, when cos(x) = 0 then sin(x) = ±1 and factors in the form we have cannot be zero.

- January 8th 2013, 12:48 PMkinhew93Re: Trigonometric equation
- January 9th 2013, 04:28 AMibduttRe: Trigonometric equation
Cos 5x + sin x = 0

OR

cos 5x + cos ( π/2- x )=0 [cos ( π/2- x )=sinx ]

2 cos (5x+ π/2- x )/2 cos (5x-( π/2- x ) )/2=0

[ Because cos C + cos D = 2 cos (C+D)/2 cos (C-D)/2]

That gives

2 cos (4x+ π/2 )/2 cos (6x- π/2 )/2=0

Now from here you can solve the equation.