Câu 1: Rút gọn các biểu thức sau:
a) \(\dfrac{sina+sin2a}{1+cosa+cos2a}\)
b) \(\dfrac{cos\left(\dfrac{\pi}{4}+a\right)-cos\left(\dfrac{\pi}{4}-a\right)}{sin\left(\dfrac{\pi}{4}+a\right)-sin\left(\dfrac{\pi}{4}-a\right)}\)
c) \(\dfrac{sin^2a}{4-4sin^2\dfrac{a}{2}}\)