tanα＝1/5
case 1: α在第1象限
sinα = 1/√26 = √26/26
cosα= 5/√26 = 5√26/26
sinα＋cosα =6√26/26 = 3√26/13
case 2: α在第3象限
sinα = -1/√26 = -1√26/26
cosα= -5/√26 = -5√26/26
sinα＋cosα =-6√26/26 = -3√26/13
