老師說把 θ角當銳角,則答案是(2),sin(1800- θ)為正,cos(1800- θ)為負。老師有舉sin(1800- θ)當例子,說明也可將 θ角視作鈍角,sin(1800- θ)依然為正,不過cos(1800- θ)就會變成正??那答案就不是(2)而是(1),為甚麼??