# How to find the exact value of cos 5pi/6?

I know that the answer to the question is  -square root 3/2, but I can't work out how to get it... can anyone help?

Posted Mon 7th May, 2012 @ 10:47 by HannahBanana

Answered Mon 7th May, 2012 @ 12:11 by Maleficent
Edited by Maleficent on Mon 7th May, 2012 @ 12:11

cos5pi/6 is the same as cos150 when converted to degrees. You also know that cos 150 is the same as -cos30. using the special triangle we know that -cos30 = A/H  so a=root3  and h=2  so    -cos30 = -root3/2.

you need to learn the special triangles, they solve everything :) i hope that helps you :)

Answered Mon 7th May, 2012 @ 14:28 by Josh Whittington

Seems you'll be needing to invest in a calculator that displays fractions... When they say 'exact value' you can almost guarantee that the answer is irrational, so they'll want an answer in fraction form :)

Answered Sat 19th May, 2012 @ 16:37 by sammiecaine

Thanks very much. The problem is that using a calculator would give me the answer as a decimal, where as I need to display the answer as an exact fraction :/

Answered Mon 7th May, 2012 @ 12:56 by HannahBanana

Get a calculator which will display fractions.

Answered Mon 7th May, 2012 @ 14:46 by Maleficent

ahh.. thank you so much, that helps a lot!! :)

Answered Sun 13th May, 2012 @ 12:31 by HannahBanana

That would work, but it is better to know how to do it without one.

Answered Mon 7th May, 2012 @ 15:11 by Josh Whittington
Edited by Josh Whittington on Mon 21st May, 2012 @ 12:56