题目

Meg. Jo, Amy and Beth live with their mother Mrs. March while their father is fighting in the Civil War. The family doesn’t have much money. They live next door to M. Laurence, who is very wealthy, and his grandson, Laurie. The passage below is about Beth, who loves to play the piano. The March family only has a small, out-of-tune piano. The “Mansion of Bliss” is Mr. Laurence’s house.But Beth, though dying for the grand piano, could not gather courage to go to the Mansion of Bliss’. She went once with Jo; but Mr. Laurence, not being aware of her weakness, stared at her so hard from u15．如图，抛物线y=$\frac{1}{2}{x}^{2}$+mx+n与x轴交于A，B两点，与y轴交于点C，抛物线的对称轴交x轴于点D，已知A（-1，0），C（0，2）．（1）求抛物线的表达式；（2）线段BC上有一动点P，过点P作y轴的平行线，交抛物线于点Q，求线段PQ的最大值．