1.sin(A+B)=sinC,sin(A+B)+sinC =2sinC不是常数,故选 C.2和3 .
另2.cos(B+C)=-cosA ,cos(B+C)+cosA =0
3.tan (A+B)/2=cotC/2,tan (A+B)/2×tanC/2=1
1.sin(A+B)=sinC,sin(A+B)+sinC =2sinC不是常数,故选 C.2和3 .
另2.cos(B+C)=-cosA ,cos(B+C)+cosA =0
3.tan (A+B)/2=cotC/2,tan (A+B)/2×tanC/2=1