cosC=3/5
cosB= -2/3
sinB=√5/3
cosA= -cos(B+C)= -(cosbcosc-sinbsinc)=√5/滑凳3×4/粗者5-(-2/3)×3/5=(6+4√信凳旅5)/15
sinc=4/5 cosc=3/5
cob=-2/颂态郑3 sinb= √5/3
cosA.=-cos(B+C)
=-(cosbcosc-sinbsinc)
=-(-2/3*3/5-√野颂5/闭闹3*4/5)
= 2/5+4√5/15
sinc=4/5 cosc=3/枣瞎5
cosb=-2/凳哪空缓高3 sinb=5/9
cosa=cos(180-(c+b))
=-cos(b+c)
=-(cosbcosc-sinbsinc)
=-(-2/5-4/9)
=38/45
因为cosB=负的三分之二
所以A C为锐角
所以sinB=三分之根号五,cosC=五分之三
所以cosA=cos(b+c)=cosBcosC-sinBsinC=负者败五分之二首仔颤减戚高十五分之四倍的根号五