REVIEW OF 10-1

Formulas for cos(a +/ B) & sin(a +/ B)

Today, I will be reteaching you lesson 10-1 where we learn the sum and difference formulas for Cosine and Sine. The formulas are as follows:

-cos(a +/- B) = cosa cosB -/+ sina sinB

-sin(a +/- B) = sina cosB +/- cosa sinB

*When using these formulas, we get our values straight off the trig chart.

Example 1: Find the exact value of sin 15 degrees.

-sin(45 - 30) = sin45cos30 - cos45sin30

-(sq root of 2/2)(sq root of 3/2) - (sq root of 2/2)(1/2)

-(sq root of 6/4) - (sq root of 2/4)

=((sq root of 6) - (sq root of 2)/4)

Example 2: Find the exact value of cos 75 degrees.

-cos(45 +30) = cos45cos30 - sin45sin30

*Now you replace the sin and cosine values with the trig chart values.

-(sq root of 2/2)(sq root of 3/2) - (sq root of 2/2)(1/2)

=((sq root of 6) - (sq root of 2)/4)

Example 3: Find the exact value of cos50cos10 - sin50sin10.

-cos(50 + 10)

-cos(60)

*Replace cos60 with its trig chart value.

=1/2