Question

已知cos(a-b)=-4/5,cos(a+b)=4/5,且(a-b)∈（π/2,π),(a+b)∈(3π/2,2π),

Answer 1

已知cos(a-b)=-4/5,cos(a+b)=4/5,且(a-b)∈（π/2,π),(a+b)∈(3π/2,2π)知sin(a+b)=-3/5,sin(a-b)=3/5,cos2a=cos[(a+b)+(a-b)]=cos(a+b)cos(a-b)-sin(a+b)sin(a-b)=-16/25-(-9/25)=-7/25cos2b=cos[(a+b)-(a-b)]=co...