Prove sin and cos = 0

Joel · #1 $Old$ November 1st, 2009, 09:14 PM

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.

adkinsjr · #2 $Old$ November 1st, 2009, 10:33 PM

Quote:

Originally Posted by Joel $View Post$

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.

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

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

The other two values are:

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

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

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"

Joel · #3 $Old$ November 2nd, 2009, 05:15 AM

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 ?

adkinsjr · #4 $Old$ November 2nd, 2009, 12:59 PM

Yup