It was the best of times, it was the worst of times,

it was the age of wisdom, it was the age of foolishness,

it was the epoch of belief, it was the epoch of incredulity,

it was the season of Light, it was the season of Darkness,

it was the spring of hope, it was the winter of despair,

we had everything before us, we had nothing before us,

we were all going direct to Heaven, we were all going direct the other way.

这是最好的时代，这是最糟的时代； 　　　　　　　　　　　　 
这是

三角形的质心：就是三条中线的交点。

例如上图：三角形ABC的三条中线为AD，BE，CF，那个他们的交点就是三角形的质心。

设质心为O，则AO/OD = 2:1, 同理可得BO/OE = 2:1, CO/OF = 2:1.

这样就可以得到质心O的坐标为：A + （AB+AC)/3，在这里AB和AC都是空间向量。