[r] How to add \newpage in Rmarkdown in a smart way?

Simply \newpage or \pagebreak will work, e.g.

hello world
\newpage
```{r, echo=FALSE}
1+1
```
\pagebreak
```{r, echo=FALSE}
plot(1:10)
```

This solution assumes you are knitting PDF. For HTML, you can achieve a similar effect by adding a tag <P style="page-break-before: always">. Note that you likely won't see a page break in your browser (HTMLs don't have pages per se), but the printing layout will have it.