some trig functions

Printable View

• Apr 25th 2009, 06:41 PM
dave30001
some trig functions
i have some questions i really need help with

sin3 A = 3sin A - 4sin^3 A
(Hint use sin3 A = sin(2a + a)

and

sin3 x / sin x - cos3 x / cos x = 2

please help im stuck (Crying)
• Apr 25th 2009, 07:33 PM
Jhevon
Quote:

Originally Posted by dave30001
i have some questions i really need help with

sin3 A = 3sin A - 4sin^3 A
(Hint use sin3 A = sin(2a + a)

um, use the hint! did you try? what did you get?

Quote:

and

sin3 x / sin x - cos3 x / cos x = 2

please help im stuck (Crying)
I will start you off:

Consider the LHS:

$\frac {\sin 3x}{\sin x} - \frac {\cos 3x}{\cos x} = \frac {\sin 3x}{\sin x} \cdot \frac {2 \cos x}{2 \cos x} - \frac {\cos 3x}{\cos x} \cdot \frac {2 \sin x}{2 \sin x}$

$= \frac {2 \sin 3x \cos x}{2 \sin x \cos x} - \frac {2 \sin x \cos 3x}{2 \sin x \cos x}$

$= \frac {2 \sin 3x \cos x - 2 \sin x \cos 3x}{\sin 2x}$

now can you finish up?