Posted by **TEETEE** on Thursday, October 25, 2012 at 9:27pm.

Use an Addition or Subtraction Formula to find the exact value of the expression.

Sin(−5π/12)

- math -
**Reiny**, Thursday, October 25, 2012 at 10:26pm
some students have less difficulty if the angle is in degrees.

Let's try this approach ....

5π/12 radians = 75°

so sin(-5π/12)

= sin(-75°) which is in quadrant IV, making the sine negative

= -sin(75°)

= -sin(30° + 45°)

= -(sin30cos45 + cos30sin45)

= -( (1/2)(√2/2) + (√3/3)(√2/2) )

= - √2/4 - √6/4

so **sin(05π/12) = -√2/4 - √6/4**

check:

on my calculator:

sin(-5π/12) = -.9659...

and -√2/4 - √6/4 = -.9659..

My answer is correct

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