Ответ:

  1. \(\cos\alpha+\cos\alpha=\boxed{2\cos\alpha}\).
  2. \(\sin\alpha-(-\sin\alpha)=\boxed{2\sin\alpha}\).
  3. \(\sin\alpha\cdot(-\sin\alpha)=\boxed{-\sin^2\alpha}\).
  4. \(-\sin\alpha+(-\cos\alpha)=\boxed{-\sin\alpha-\cos\alpha}\).
  5. \((-\sin\alpha)\cdot\sin\alpha=\boxed{-\sin^2\alpha}\).