若方程ax²=b（ab>0）的两个根分别是2m+5与4m+1，则$\frac{b}{a}$的值为（  ）        

①cos（A+B）=cosC

②sin（2A+B+C）=sinA

③ $cos\frac{B+C}{2}=sin\frac{A}{2}$

④tan（A+B）=﹣tanC．

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