A,B为锐角sinA=√5/5cosA=√[1-(sinA)^2=2√5/5tanB=3sinB/cosB=3(sinB)^2+(cosB)^2=1所以sinB=3/√10cosB=1/√10cos(A-B)=cosAcosB+sinAsinB=√2/2cos(A+B)=cosAcosB-sinAsinB=-√2/10
