一个三角形的所有角度之和是180度。

The sum of all the angles of a triangle is 180 degrees.

《柯林斯英汉双解大词典》

设x为a加b之和。

Let x be the sum of a and b.

《牛津词典》

youdao

Doing that in terms of the contents gives you a subtotal, but that subtotal is no longer the entire story.

把生活内容相加能得到部分之和，但是这个部分之和不再是整体。

耶鲁公开课 - 死亡课程节选

It has to be part of a theory, and then trying to adjust it so it can be incorporated, you discover anti-particles.

要想使理论合理,然后试着去调整,使之和实际相符,结果就找到了反粒子

耶鲁公开课 - 基础物理课程节选

And what's important to keep in mind about formal charge is if we have a neutral atom, such as we did in thionyl chloride here, the sum of the individual formal charges on individual atoms within the molecule have to equal 0.

而关于形式电荷记住一点是很重要的,那就是如果我们有一个中性原子，比如亚硫酰氯，那么这个分子中的所有原子的,形式电荷之和应该等于零。

麻省理工公开课 - 化学原理课程节选