红色羊毛衫_
幼苗
共回答了17个问题采纳率：100% 举报
∵AB＝4I－2J=(4,-2)
AC＝7I＋4J=(7,4)
AD＝3I＋6J=(3,6)
∴|AB|=√(16+4)=2√5
|AD|=√(9+36)=3√5
∵cos∠ABC=AB.AD/(|AB|×|AD|)
=(4,-2).(3,6)/[(2√5)×(3√5)]
=(12-12)/[(2√5)×(3√5)]
=0
∴∠ABC=90°
∴SABCD=2S△ABC=2AB×BC/2
=2[(2√5)×(3√5)]/2
=30
1年前
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