解：ab=lallblcos45°=√2 bc=lbllclcos45°=3√2 ac=lallclcos45°=(3/2)√2 |a+b-c|=√(a+b-c)²=√(a²+2ab+b²-2ac-2bc+c²)=√(1+2√2+4-3√2-6√2+9)=√(14-7√2)
