英语翻译In the xy-coordinate plane,P and Q are different points

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英语翻译
In the xy-coordinate plane,P and Q are different points that have the same y-coordinate and lie on the parabola whose equation is y=x^2-3x-40.What is the x-coordinate of the midpoint of PQ?

在xy平面里,P、Q为纵坐标相同的两点,并且P、Q两点在抛物线y=x^2-3x-40上.求线段PQ中点的横坐标.
自己参考吧：Xm=1/2(Xp+XQ).利用抛物线的symmetry（对称性）,Xm=1/2*2*[-(-3/2)]=1.5
再问： Xm=1/2*2*[-(-3/2)]=1.5 = =这个是怎么带出来的。。。
再答：首先由于对称性，P，Q两点的横坐标之和是对称轴横坐标的两倍。而对称轴横坐标是-b/2a，即-(-3)/（2*1）(把二次函数写成f（x）=ax^2+bx+c(a~=0)).那么整个就是1/2*2*[-(-3/2)]=1.5. 或者这样想，由于对称，那么PQ中点就在抛物线对称轴上，那么其横坐标就是对称轴横坐标，即-(-3)/(2*1)=1.5

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