2025年勤学早九年级数学上册人教版第51页答案
1. 二次函数$y = ax^{2}+bx + c$的图象如图所示,回答下列问题:
(1) 抛物线开口____,$a$____$0$;
(2) 抛物线的对称轴在$y$轴____,$-\frac{b}{2a}$____$0$,$\therefore b$____$0$;
(3) 抛物线与$y$轴负半轴相交,则$c$____$0$,$\therefore abc$____$0$;
(4) 抛物线与$x$轴有____个不同的交点,$\therefore \Delta = b^{2}-4ac$____$0$.

答案

(1)向上 > (2)右侧 > < (3)< > (4)两 >
2. (原创题)已知二次函数$y = ax^{2}+bx + c$的图象如图所示,顶点是$(1,-4)$,回答下列问题:
(1) 方程$ax^{2}+bx + c = -3$的解为____;
(2) 方程$ax^{2}+bx + c = -4$的解为____;
(3) 当$y > 0$时,$x$的取值范围是____;
(4) 当$y < -3$时,$x$的取值范围是____;
(5) 当$-3 < y < 0$时,$x$的取值范围是____.

答案

(1)$x_{1}=0,x_{2}=2$ (2)$x_{1}=x_{2}=1$ (3)$x<-1$或$x>3$ (4)$0<x<2$ (5)$-1<x<0$或$2<x<3$
3. (原创题)已知二次函数$y = ax^{2}+bx + c$的图象如图所示,回答下列问题:
(1)$a$____$0$; (2)$b$____$0$; (3)$c$____$0$;
(4)$a + b + c$____$0$;(5)$a - b + c$____$0$;(6)$2a + b$____$0$;
(7) 方程$ax^{2}+bx + c = 0$的根为____;
(8) 当$y > 0$时,$x$的取值范围为____;
(9) 当$y < 0$时,$x$的取值范围为____.

答案

(1)< (2)> (3)> (4)> (5)= (6)= (7)$x_{1}=3,x_{2}=-1$ (8)$-1<x<3$ (9)$x<-1$或$x>3$
4. 二次函数$y = ax^{2}+bx + c$的图象如图所示,回答下列问题:
(1)$a$____$0$,$b$____$0$,$c$____$0$,$abc$____$0$;
(2) 当$x = 1$时,$y$____$0$,$\therefore a + b + c$____$0$;
(3) 当$x = -2$时,$y$____$0$,$\therefore 4a - 2b + c$____$0$;
(4) 当$x = 2$时,$y$____$0$,$\therefore 4a + 2b + c$____$0$;
(5) 当$x = -3$时,$y$____$0$,$\therefore 9a - 3b + c$____$0$;
(6) 对称轴为直线$x = $____,$-\frac{b}{2a}= $____,$\therefore b$____$2a$;
(7)$3a + c$____$0$,$4b + c$____$0$.

答案

(1)< < > > (2)< < (3)> > (4)< < (5)< < (6)-1 -1 = (7)< <
5. 如图,抛物线$y = ax^{2}+bx + c的对称轴是直线x = 1$. 回答下列问题:
(1)$abc$____$0$; (2)$b^{2}-4ac$____$0$;
(3)$4a - 2b + c$____$0$; (4)$8a + c$____$0$.

答案

(1)< (2)> (3)< (4)< 提示:$\because -\frac {b}{2a}=1,$ $\therefore b=-2a.$ 又$\because x=-2$时,$y=4a-2b+c<0,$ $\therefore 4a-2(-2a)+c<0$,即$8a+c<0.$