5.
我们定义:如图\(1\),在\(\Delta ABC\)看,把\(AB\)点\(A\)顺时针旋转\(\alpha \left( {{0}^{0}} < \alpha < {{180}^{0}} \right)\)得到\(A{B}{{{'}}}\),把\(AC\)绕点\(A\)逆时针旋转\(\beta \)得到\(A{C}{{{'}}}\),连接\({B}{{{'}}}{C}{{{'}}}.\)当\(\alpha +\beta ={{180}^{0}}\)时,我们称\(\Delta {A}{{{'}}}{B}{{{'}}}{C}{{{'}}}\)是\(\Delta ABC\)的“旋补三角形”,\(\Delta A{B}{{{'}}}{C}{{{'}}}\)边\({B}{{{'}}}{C}{{{'}}}\)上的中线\(AD\)叫做\(\Delta ABC\)的“旋补中线”,点\(A\)叫做“旋补中心”.
特例感知:
\((1)\)在图\(2\),图\(3\)中,\(\Delta A{B}{{{'}}}{C}{{{'}}}\)是\(\Delta ABC\)的“旋补三角形”,\(AD\)是\(\Delta ABC\)的“旋补中心”.
\(①\)如图\(2\),当\(\Delta ABC\)为等边三角形时,\(AD\)与\(BC\)的数量关系为\(AD=\)_____________\(BC\);
\(②\)如图\(3\),当\(\angle BAC={{90}^{0}},BC=8\)时,则\(AD\)长为_________________.
猜想论证:
\((2)\)在图\(1\)中,当\(\Delta ABC\)为任意三角形时,猜想\(AD\)与\(BC\)的数量关系,并给予证明.
拓展应用
\((3)\)如图\(4\),在四边形\(ABCD\),\(\angle C={{90}^{0}},\angle D={{150}^{0}},BC=12\),\(CD=2\sqrt{3},DA=6.\)在四边形内部是否存在点\(P\),使\(\Delta PDC\)是\(\Delta PAB\)的“旋补三角形”?若存在,给予证明,并求\(\Delta PAB\)的“旋补中线”长;若不存在,说明理由.