Posted by **Calculus (trig stuff)** on Sunday, October 25, 2009 at 9:33pm.

How do I do these type of problems

find the exact value. Do not use a calculator

cos 7pi

- Calculus (trig stuff) -
**michael**, Monday, October 26, 2009 at 12:24am
2&pi is one full rotation around the circle, so you can think of 2&pi as being the same as 0. so 7 pi is the same as going around the circle 3 full times with 1&pi left over, to figure this out exactly you can do the following:

(7/2) = 3 +1/2

the 3 is how many times we went 2&pi around the circle, and what we have left is (1/2)*2&pi or 1&pi

cos(&pi) = -1

To do another example

sin((9/2)&pi)

well (9/2)/2 is the same as (9/2)*(1/2)=(9/4)

we get 2 + 1/4 so we get 2 full rotations plus 1/4 of a full rotation. A full rotation is just 2&pi so:

2&pi*(1/4) = &pi/2

sin(&pi/2) = 1

## Answer this Question

## Related Questions

- math - Eliminate the parameter (What does that mean?) and write a rectangular ...
- Pre-Calc - I am really struggling with how to do these problems, I posted them a...
- Trigonometry - How do you find the exact value of cos -7pi/12 without using a ...
- Trigonometry - Hi, I need help with this question: Use the addition and ...
- trig - Evaluate giving exact values and identities. Calculator based solutions ...
- Pre-Calc - 1) Use an Addition or Subtraction Formula to write the expression as ...
- trig - Find the exact value. Without using a calculator a) cos (-17pi/6) b) sec...
- Math - Find the exact value: 1.) cos^-1(sin(pi/2)) 2.) arccos(cos(7pi/2))
- Math - Find the exact value: 1.) cos^-1(sin(pi/2)) 2.) arccos(cos(7pi/2))
- math - Determine the exact value of cos^-1 (pi/2). Give a number and an ...