This is a shorthand, I'm going to write it only once,

这是个专用符号我只写一遍
but you should know what the notation means.

但你们应该理解这种记法的意思
If there is i from 1 to 3,

如果 i 从 1 取到 3
it really means m1·x1 + m2·x2 + m3·x3

这个式子实际等于 m1·x1 + m2·x2 + m3·x3
divided by m1 + m2 + m3.

除以 m1 + m2 + m3
This summation is the notation mathematicians have introduced

这种求和的表示方法是从数学引入的
where the index I will go over a range from 1 to 3.

下标 i 会从 1 取到 3
Every term you let i take three different values and you do the sum.

你们让 i 分别取三个值然后让它们相加
An exercise I've given to you guys to pursue at home is the following:

我现在给你们布置课后思考题
if I got three bodies, 1, 2 and 3,

如果我有三个物体 1 2 和 3
you've defined the center of mass,

你们已经定义了质心
you can either go to this formula,

你们可以使用这个公式