# trig question

• Aug 26th 2007, 07:30 AM
gregorio
trig question
if i have cos theta = 0.4
what are the values of

cos (180-theta)
cos (360-theta)

i solve like this
cos.4 inverse = 66.42
therefore cos ( 180-66.42)
i have cos *113.57 is this answer or must i make multiplication which gets me -.4 ??
• Aug 26th 2007, 07:38 AM
Jhevon
Quote:

Originally Posted by gregorio
if i have cos theta = 0.4
what are the values of

cos (180-theta)
cos (360-theta)

i solve like this
cos.4 inverse = 66.42
therefore cos ( 180-66.42)
i have cos *113.57 is this answer or must i make multiplication which gets me -.4 ??

there are several ways to approach this

you can note that:

$\cos (180 - \theta ) = - \cos \theta$

and

$\cos (360 - \theta ) = \cos \theta$

OR

you can do it the way you did, and you would get the same answer

OR

you can use the double angle formula:

$\cos (A - B) = \cos A \cos B + \sin A \sin B$ ........which, by the way, is how we know the first two formulas i gave you

and, by the way, it is NOT cos * 113.57, it is cos(113.57). the cosine function is an argument, you cannot treat it like a term unto itself and multiply it with something