∴b2+c2-a2=bc,
∴
b2+c2−a2 |
2bc |
1 |
2 |
∴cosA=
1 |
2 |
又A是三角形的内角,故A=
π |
3 |
(2)∵2sin2
B |
2 |
C |
2 |
∴1-cosB+1-cosC=1∴cosB+cosC=1,
由(1)的结论知,A=
π |
3 |
2π |
3 |
∴cosB+cos(
2π |
3 |
即cosB+cos
2π |
3 |
2π |
3 |
即
| ||
2 |
1 |
2 |
∴sin(B+
π |
6 |
又0<B<
2π |
3 |
π |
6 |
π |
6 |
5π |
6 |
∴B+
π |
6 |
π |
2 |
∴B=
π |
3 |
π |
3 |
故△ABC是等边三角形.
B |
2 |
C |
2 |
b2+c2−a2 |
2bc |
1 |
2 |
1 |
2 |
π |
3 |
B |
2 |
C |
2 |
π |
3 |
2π |
3 |
2π |
3 |
2π |
3 |
2π |
3 |
| ||
2 |
1 |
2 |
π |
6 |
2π |
3 |
π |
6 |
π |
6 |
5π |
6 |
π |
6 |
π |
2 |
π |
3 |
π |
3 |