△ABC=△ABD+△ACDより、
1/2×AB×AC×sin120ﾟ
=1/2×AB×AD×sin60ﾟ+1/2×AC×AD×sin60ﾟ
6×3×√3/2=6×AD×√3/2+3×AD×√3/2
18=6AD+3AD
9AD=18
AD=2
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