5sinA=2sinC+ sinB
因为
a/sinA=b/sinB=c/sinC=2r
所以
sinA=a/2r
sinB=b/2r
sinC=c/2r
所以
5a/2r=2c/2r+b/2r
5a=2c+b
40=28+b
b=12
祝开心