cos(a+b)推导,cos(α-β)推导过程

cosa=1/3,cos(a b)=-1/3,且a b属于(0,π/2),则cos(a
怎么用余弦公式cos(a b)=cosa*cosb-sina*sinb和诱导公式5,6推出sin(a
b b cos a,b
0≤a b r 2π 且cos cosb cosr=sina sinb sinr=0 求b
化简:cos(a b)cosa sin(a b)sina