1) Cos ( alpha + or - B) = Cos alpha Cos B + or - Sin alpha Sin B
2) Sin ( alpha + or - B) = Sin alpha Cos B + or - Cos alpha Sin B
3) Sin x + Sin y = 2 Sin x+y/2 Cos x-y/2
4) Sin x - Sin y = 2 Cos x+y/2 Sin x-y/2
5) Cos x = Cos y = 2 Cos x+y/2 Cos x-y/2
6) Cos x - Cos y = -2 Sin x+y/2 Sin x-y/2
*Don't plug into formula
*You have to replace
*Refer back to trig chart to find exact values
Example 1: Find the exact value of Sin 15 degrees
Sin (45- 30) = Sin 45 Cos 30 - Cos 45 Sin 30
= square root of 3/2 * square root of 3/2 - square root of 2/2 * 1/2
= square root of 6/9 - square root of 2/4 = square root of 6- square root of 2 /4
1) Refer back to angles on your trig chart and convert 15 into ( 45- 30)
2) Find out which formula you have to use.
3) Replace by using trig chart
4) Multiply the top by top and bottom by bottom.
5) Subtract both your answers together, and keep the denominator because it is common.
