Question

如图,已知抛物线y=-x^2+bx+c与x轴的两个交点分别为A(x1,0)、B(x2,0),且x1+x2=4,x2分之x

Answer 1

1

x1+x2=-b/a

b=4

y=-x²+4x+c

x1/x2=1/3

3x1=x2

3x1=4-x1

x1=1 x2=3

-1+4+c=0

c=-3

y=-x²+4x-3

A(1,0) B(3,0) C(0,-3)

2

BC

(y+3)/3=x/3

y=x-3

3

S△ABC=(3-1)*3/2=3

4

-x²+4x-3=3

无解

-x²+4x-3=-3

x=0 x=4

D(4,-3)