b²+c²=6(b+c)-18
b²-6b+c²-6c+18=0
b²-6b+9+c²-6c+9=0
(b-3)²+(c-3)²=0
平方項(xiàng)恒非負(fù),兩非負(fù)項(xiàng)之和=0,兩非負(fù)項(xiàng)分別=0
b=3 c=3
b=c ∠B=∠C
∠A+∠B+∠C=180°
∠B=∠C=(180°-∠A)/2=(180°-60°)/2=60°=∠A
a=b=c
a=3