# Thread: Prove sin and cos = 0

1. ## Prove sin and cos = 0

Hi all,

sin330cos150 - cos390sin390 = 0

Prove:

What am I to do with this? Do I convert ll to the one function or......mmmm got nothing.

Any help would be appreciated.

2. Originally Posted by Joel
Hi all,

sin330cos150 - cos390sin390 = 0

Prove:

What am I to do with this? Do I convert ll to the one function or......mmmm got nothing.

Any help would be appreciated.
Remember that the trig functions are periodic.

$\displaystyle cos(390)=cos(30)=\frac{\sqrt{3}}{2}$

$\displaystyle sin(390)=sin(30)=\frac{1}{2}$

The other two values are:

$\displaystyle sin(330)=-\frac{1}{2}$

$\displaystyle cos(150)=-\frac{\sqrt{3}}{2}$

If you don't see why these are true, look here: Trigonometric functions - Wikipedia, the free encyclopedia

Scroll down and look at the unit circle next to the paragraph "Unit-circle definitions"

3. so it would now look like....

sin330cos150 - cos390sin390 = 0

(- ½)(-√3/2) - ( ½)(√3/2) = 0
√3/2 / 2 - √3/2 / 2 = 0 which is true

Is this how i should set it out ?

4. Yup