三角形abc中,角A,B,C所对的边为a b c ,cosA=5分之4,若b=2三角形的面积为3 求tanC

B+C=π-A
sin^2(B+C)/2+cos2A
=sin^2[π/2-A/2]+cos2A
=cos^2(A/2)+cos2A
=(1+cosA)/2+2cos^2A-1 cosA=4/5
=(1+4/5)/2+32/25-1
=9/10+7/25
=59/50
打字不易,如满意,望采纳.