将解析式化为顶点坐标的形式
y=ax^2+bx+c
=a(x^2+bx/a)+c
=a[(x+b/2a)^2-(b/2a)^2]+c
=a(x+b/2a)^2 - b^2/4a +c
=a(x+b/2a)^2+(b^2-4ac)/4a
所以 二次函数y=ax^2+bx+c(a,b,c为常数,a≠0)的
顶点是(-b/2a,(b^2-4ac)/4a)
对称轴是 x= -b/2a
-b/2a=2
b=-4a
y=ax^2-4ax+c
0=a+4a+c
c=-5a
y=ax^2-4ax-5a
y=a(x^2-4x)-5a
y=a(x^2-4x+4)-5a-4a
y=a(x-2)^2-9a
0=a(x-2)^2-9a
(x-2)^2-9=0
x^2-4x-5=0
(x-5)(x+1)=0
x=5或x=-1
即C点座标为:(5,0)
S=[5-(-1)]*∣-9a∣/2
=3∣-9a∣
18=3∣-9a∣
6=∣-9a∣
a=±2/3
当a=2/3时 b=-8/3 ,c=-10/3
y=2x^2/3-8x/3-10/3
当a=-2/3时 b=8/3 ,c=10/3
y=-2x^2/3+8x/3+10/3
所以y=2x^2/3-8x/3-10/3或y=-2x^2/3+8x/3+10/3