Look in the index and/or glossary for these formulas.
Use the addition formula for sine.
sin(50° + 20°)
Use the sum/difference formula for the cosine. Be careful of your sign.
Use the Half-Angle Formula for cosine.
Use the Double-Angle Formula for the sine.
Use one of the Double-Angle Formulas for the cosine.
Use the Half-Angle Formula for sine.