∴acosC+ccosA=2bcosB,
由正弦定理得,a=2RsinA,b=2RsinB,c=2RsinC,
代入得:2RsinAcosC+2RcosAsinC=4RsinBcosB,
即:sin(A+C)=sinB,
∴sinB=2sinBcosB,
又在△ABC中,sinB≠0,
∴cosB=
1 |
2 |
∵0<B<π,
∴B=
π |
3 |
(Ⅱ)∵B=
π |
3 |
∴A+C=
2π |
3 |
∴2sin2A+cos(A-C)=1-cos2A+cos(2A-
2π |
3 |
=1-cos2A-
1 |
2 |
| ||
2 |
| ||
2 |
3 |
2 |
=1+
3 |
π |
3 |
∵0<A<
2π |
3 |
π |
3 |
π |
3 |
∴-
| ||
2 |
π |
3 |
∴2sin2A+cos(A-C)的范围是(-
1 |
2 |
3 |