PDF Output with Knitr for Latex Equations - pdf

Question

I am having trouble getting multi-line latex equations to successfully compile when using knitr in R Markdown. When I output as an HTML file it works but when I attempt to output as a PDF I get errors and it will not compile.
The following sample code illustrates the problem. Just change output:html_document to output: pdf_document. You'll see that in HTML output both Test 1 and Test 2 work. But in PDF output only Test 1 successfully compiles.
---
title: "Test"
date: "1/16/2018"
output: html_document
---
## R Markdown
Testing to see if a multi-line latex equation works.
#### Test 1:
$$ f(x) = x^5 + x^3 + x $$
$$ g(x) = y^{x+1} $$
#### Test 2:
\begin{align}
f(x) &= x^5 + x^3 + x \\
g(x) &= y^{x+1}
\end{align}
I am running Mac OS X - High Sierra and R Studio - Version 1.1.383
The following is the error I got:
Rule 'pdflatex': File changes, etc:
Non-existent destination files:
'Untitled.pdf'
------------
Run number 1 of rule 'pdflatex'
------------
------------
Running 'pdflatex -halt-on-error -interaction=batchmode -recorder "Untitled.tex"'
------------
This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017) (preloaded format=pdflatex)
restricted \write18 enabled.
entering extended mode
=== TeX engine is 'pdfTeX'
Latexmk: Errors, so I did not complete making targets
Collected error summary (may duplicate other messages):
pdflatex: Command for 'pdflatex' gave return code 1
Refer to 'Untitled.log' for details
Latexmk: Use the -f option to force complete processing,
unless error was exceeding maximum runs of latex/pdflatex.
! Paragraph ended before \align was complete.
<to be read again>
\par
l.112
Error: Failed to compile Untitled.tex. See Untitled.log for more info.
Execution halted