# math

posted by
**Bill** on
.

Find the exact value of cos 300 degrees.

thanks guys

cos 300 = 1/2 = 0.500

how do you know? I am supposed to show my work.

You ought to know the rule on 30-60-90 triangles. If the hyp is 2, the shorter side is 1, and the longer side is sqrt3.

what does that have to do with cos300?

Cos(x) = Cos(-x) (1)

Cos(x + 360) = cos(x) (2)

Cos(300) = (using (2)) cos(-60) = (using (1)) cos(60) = using what Bobpursley wrote above 1/2

where did you get 60 from?

Bill, the sixty came from the addition formula above. 300 + 60 = 360

Bob, it says Cos(x + 360) .

Cos(x + 360) = Cos(x)

substitute x = y - 360:

Cos(y - 360 + 360) = Cos(y - 360) --->

Cos(y) = Cos(y - 360)

Substitute y = 300:

cos(300) = cos(-60)

Where did you get substitute x = y - 360 from? I'm sorry, I am so confused. Do you think you could give me the formula or whatever and then show me how to plug in the 300, or is that what you are doing now?