The value of cos70° + cos50° − cos10° is:

Sum to product on the first two: cos70° + cos50° = 2·cos60°·cos10° = 2 × (1/2) × cos10° = cos10°, which the third term cancels exactly. Check: 0.3420 + 0.6428 − 0.9848 = 0. Forgetting that the factor 2cos60° is just 1 leaves the answer stranded at 2cos10° or cos10°.