由图像的顶点是（1,4）得x=-b/2a=1,y=(4ac-b²)/4a=4,由图像经过点（2,8）得4a+2b+c=0;解上面方程:a=-b/2 -(2bc+b²)/(-2b)=4 ,c=4-b/2-2b+2b+4-b/2=0,b=8 ,a=-4,c=o,二次函数解析式为:y=-4x²+8x...