sin(A-B)cosA-cos(A-B)sinA = sin(A-B-A)=sin(-B)= - sinB=√7/4,即sinB= - √7/4,又B在第三象限,则cosB也是负值,由(sinB)^2+(cosB)^2=1,得(cosB)^2 = 1-(sinB)^2= 1 -7/16 = 9/16,已知cosB也是负值,则cosB= -3/4...