题目内容：

在三角形ABC中,AD是中线,AE是高.求证:AB平方-AC平方=2BC*DE
在三角形ABC中,AD是中线,AE是高.求证:(1)AB平方-AC平方=2BC*DE(2)已知:AB=6,BC=5,AC=4,求DE的长
Betty is the most careful girl in her class.（同义句）
Betty is more careful than the other girls in her
class
2,Betty is （5个空）girl in her class.

优质解答

AB²=AE²+BE² AC²=AE²+CE² AB²-AC² =BE²-CE² =(BE+CE)(BE-CE) =BC*[(BD+DE)-(CD-DE)] =BC*(2*DE) =2*BC*DE 6^2-4^2=2*5*DE DE=2行不行?