Question

已知抛物线y=a乘以x的平方+2x+c的顶点坐标是（3分之1,-1）,则a乘以c=

Answer 1

对称轴x=-1/a=1/3

a=-3

y=-3x^2+2x+c=-3(x-1/3)^2+c+1/3

c+1/3=-1

c=-4/3

a*c=(-3)*(-4/3)=4