满意请给好评 AB=(2,2,2) AC=(1,2,4) BC=(-1,0,2) |AB|=2√3 |AC|=√21 |BC|=√5 cosA=(12+21-5)/(2*2√3*√21)=√7/3 sinA^2=1-cosA^2=1-7/9=2/9 sinA=√2/3 所以s=（1/2）*2√3√21*√2/3=√14
