总结R中两个表中的值

我有两个表格如下：

A<-data.frame("Task"=c("a","b","c","d","e"),"FC"=(c(100,120,200,300,400)))
B<-data.frame("Task"=c("a","b","c"),"FC"=(c(20,50,30)))

  Task  FC
1    a 100
2    b 120
3    c 200
4    d 300
5    e 400
  Task FC
1    a 20
2    b 50
3    c 30

合并dfs

df=merge(A,B,by="Task",all=F)

总结数据

df$sum=apply(df[,2:3],1,sum)#sum, sd, min, max or ...

> df
  Task FC.x FC.y sum
1    a  100   20 120
2    b  120   50 170
3    c  200   30 230

---

这个汇总值到底是多少？看起来像个总数？

df$sum=apply(df[,2:3],1,sum)#sum, sd, min, max or ...

> df
  Task FC.x FC.y sum
1    a  100   20 120
2    b  120   50 170
3    c  200   30 230