(1)m、n共线--->(cosB/2):(1/2) = (1/2):(cosB/2)
--->cos²(B/2)=1/4--->cos(B/2)=1/2(B/2为锐角,负值舍去)
--->B/2=60°--->B=120°
(2)--->A+C=60°
cosC=3/5--->sinC=4/5
cosA=cos(60-C)=cos60cosA+sin60sinA=(3+4√3)/10
(1)m、n共线--->(cosB/2):(1/2) = (1/2):(cosB/2)
--->cos²(B/2)=1/4--->cos(B/2)=1/2(B/2为锐角,负值舍去)
--->B/2=60°--->B=120°
(2)--->A+C=60°
cosC=3/5--->sinC=4/5
cosA=cos(60-C)=cos60cosA+sin60sinA=(3+4√3)/10