Let's say I have cos(5pi/3). How do I know this equals 0.5 without using a calculator?

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- Sep 15th 2010, 09:17 AMjayshizwizHow do you convert from cos(pi/number)
Let's say I have cos(5pi/3). How do I know this equals 0.5 without using a calculator?

- Sep 15th 2010, 09:36 AMyeKciM
- Sep 15th 2010, 09:53 AMjayshizwiz
thanks but hopefully someone has a better explanation (:

and you wrote pi/6 twice...

I know how to do it I guess by converting it from radian to degrees: cos(5pi/3) = cos300=cos60=0.5

but isn't there a way to measure it by radian? - Sep 15th 2010, 10:00 AMyeKciM
hehehehe... while pasting forgot to change that pi/3 :D:D:D

but if you draw your self that angle ... you'll realize that angle of is the same as the angle of and you know that angle that is

because cos is positive in first and 4th quadrant :D:D:D:D get it ?

when drawn circle, draw and add it 5 times and you'll see that is the angle :D

P.S. you realize that so is just for the smaller than so it have to be - Sep 15th 2010, 10:09 AMundefined
- Sep 15th 2010, 01:15 PMebaines
The key concept her is to realize that 1 radian is the angle that is covered if you wrap a string of length 1 x radius around the perimeter of a circle. By convention we usually just consider a circle of radius 1, which would have a total perimeter length 2 pi. Hence a string of length 2 pi would wrap precisely once around this unit circle, and so the angle 2 pi radians is eqivalent to 360 degrees. Similarly, pi/2 radians is 180 degreees, pi/3 radians = 120 degrees, etc. Now, when first learning trig there is a natural tendency for some students to resist thinking in radians, and try to convert everything to degrees in their head. But trust me - you'd be much beter off getting to a point where you can "visualize" what pi/4 radians means without first thinking about degrees. Just consider that each quadrant of the circle is pi/2 radians, and so 2/3 pi is in the second quadrant, 5/3 pi is in the fourth, etc. So get used to visualizing how it is that tan(pi/4) = 1, and you'll be well on your way.

- Sep 15th 2010, 04:19 PMArchie Meade
Without using a calculator...

That's

Hence, if you draw a unit-circle centred at the origin, you can draw a right-angled triangle

with hypotenuse going off at 60 degrees to the x-axis,

since Cos(angle) gives the x co-ordinate, and Cos(-A)=CosA.

This allows us to draw a regular hexagon, made of equilateral triangles as shown in the attachment.

From that, we can see that