15. (2025·江苏南京模拟·15分)新素养
几何直观 如图,在平面直角坐标系中,O是原点,抛物线$L_1:y=x^2+bx+c$经过点$C(0,-3)$,与抛物线$L_2:y=-\frac{1}{2}x^2-\frac{3}{2}x+2$的一个交点为A,且点A的横坐标为2,P,Q分别是抛物线$L_1,L_2$上的动点.
(1)求抛物线$L_1$的函数表达式;
(2)若以A,C,P,Q四点为顶点的四边形恰为平行四边形,求点P的坐标;
(3)设R为抛物线$L_1$上另一个动点,且CA平分$\angle PCR$.若$OQ// PR$,求点Q的坐标.

答案:15.(1)在$y = -\frac{1}{2}x^{2}-\frac{3}{2}x + 2$中,令$x = 2$,得$y = - 3$,所以$A(2,-3)$.把点$A(2,-3)$,$C(0,-3)$分别代入$y = x^{2}+bx + c$,得$\begin{cases}4 + 2b + c = - 3,\\c = - 3,\end{cases}$解得$\begin{cases}b = - 2,\\c = - 3,\end{cases}$所以抛物线$L_{1}$的函数表达式为$y = x^{2}-2x - 3$.
(2)设$P(m,m^{2}-2m - 3)$,$Q(n,-\frac{1}{2}n^{2}-\frac{3}{2}n + 2)$.又$A(2,-3)$,$C(0,-3)$,所以当以$A$,$C$,$P$,$Q$四点为顶点的四边形恰为平行四边形时,分类讨论如下:①若$AC$为平行四边形的对角线,则$\begin{cases}m + n = 2,\\m^{2}-2m - 3+(-\frac{1}{2}n^{2}-\frac{3}{2}n + 2)= - 3+(-3),\end{cases}$解得$\begin{cases}m = - 3,\ = 5\end{cases}$或$\begin{cases}m = 0,\ = 2\end{cases}$(不合题意,舍去),则$m^{2}-2m - 3 = 12$,所以$P(-3,12)$;②若$AP$为平行四边形的对角线,则$\begin{cases}m + 2 = n,\\m^{2}-2m - 3+(-3)=-\frac{1}{2}n^{2}-\frac{3}{2}n + 2+(-3),\end{cases}$解得$\begin{cases}m = - 1,\ = 1\end{cases}$或$\begin{cases}m = 0,\ = 2\end{cases}$(不合题意,舍去),则$m^{2}-2m - 3 = 0$,所以$P(-1,0)$;③若$AQ$为平行四边形的对角线,则$\begin{cases}n + 2 = m,\\-\frac{1}{2}n^{2}-\frac{3}{2}n + 2+(-3)=m^{2}-2m - 3+(-3),\end{cases}$解得$\begin{cases}m = 3,\ = 1\end{cases}$或$\begin{cases}m = -\frac{4}{3},\ = -\frac{10}{3}\end{cases}$当$m = 3$时,$m^{2}-2m - 3 = 0$;当$m = -\frac{4}{3}$时,$m^{2}-2m - 3 = \frac{13}{9}$.所以$P(3,0)$或$P(-\frac{4}{3},\frac{13}{9})$.综上所述,点$P$的坐标为$(-3,12)$或$(-1,0)$或$(3,0)$或$(-\frac{4}{3},\frac{13}{9})$.
(3)如图,当点$P$在$y$轴左侧时,抛物线$L_{1}$上不存在点$R$,使得$CA$平分$\angle PCR$;当点$P$在$y$轴右侧时,不妨设点$P$在$CA$的上方,点$R$在$CA$的下方.过$P$,$R$两点作$y$轴的垂线,垂足分别为$S$,$T$,过点$P$作$PH⊥ TR$交$TR$的延长线于点$H$,则$\angle PSC = \angle RTC = 90^{\circ}$.因为$A(2,-3)$,$C(0,-3)$,所以$AC// x$轴,所以$AC⊥ y$轴,所以$\angle ACS = \angle ACT = 90^{\circ}$.因为$CA$平分$\angle PCR$,所以$\angle PCA = \angle RCA$.又$\angle PCA+\angle PCS = \angle ACS$,$\angle RCA + \angle RCT = \angle ACT$,所以$\angle PCS = \angle RCT$,所以$\triangle PSC∼\triangle RTC$,所以$\frac{PS}{CS}=\frac{RT}{CT}$.设$P(x_{1},x_{1}^{2}-2x_{1}-3)$,$R(x_{2},x_{2}^{2}-2x_{2}-3)$,则$\frac{x_{1}}{x_{1}^{2}-2x_{1}-3-(-3)}=\frac{x_{2}}{-3-(x_{2}^{2}-2x_{2}-3)}$.整理,得$x_{1}+x_{2} = 4$.在$Rt\triangle PRH$中,$\tan\angle PRH = \frac{PH}{RH}=\frac{x_{1}^{2}-2x_{1}-3-(x_{2}^{2}-2x_{2}-3)}{x_{1}-x_{2}}=x_{1}+x_{2}-2 = 2$.过点$Q$作$QK⊥ x$轴于点$K$.设$Q(t,-\frac{1}{2}t^{2}-\frac{3}{2}t + 2)$.若$OQ// PR$,则$\angle QOK = \angle PRH$,所以$-\frac{1}{2}t^{2}-\frac{3}{2}t + 2 = \tan\angle PRH = 2$,所以$-\frac{1}{2}t^{2}-\frac{3}{2}t + 2 = - 7\pm\sqrt{65}$,解得$t = \frac{-7\pm\sqrt{65}}{2}$,则$-\frac{1}{2}t^{2}-\frac{3}{2}t + 2 = - 7\pm\sqrt{65}$,所以点$Q$的坐标为$(\frac{-7+\sqrt{65}}{2},-7+\sqrt{65})$或$(\frac{-7-\sqrt{65}}{2},-7-\sqrt{65})$.
