A problem of quadratic function in junior high school Yingxian bridge, rebuilt in 1844, is located on the ancient trunk road between Zhejiang and Fujian. According to taoshuwu village, 15km southeast of Xinchang County, the bridge is a single hole parabolic stone arch bridge with smooth arch. The span and height of the arch are known to be 15.6m and 7.7m respectively. An appropriate plane rectangular coordinate system is established and the quadratic function relationship corresponding to the parabola is obtained

Take the center point of the bridge arch as the coordinate origin. The span of the bridge arch is the x-axis and the height of the arch is the y-axis. The coordinates of the three points known in the title are the left Pier (- 7.8,0), the bridge vertex (0,7.7) and the right Pier (7.8,0). Suppose that the quadratic function of the parabola is y = ax ^ 2 + BB = 7.77.8 ^ 2A + 7.7 = 0A = -0.13

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