"If two sinusoids with much different periods are combined to form a periodic function with a varying sinusoidal axis, which operation was used?"
Response:
Multiplication.
"If two sinusoids with much different periods are combined to form a periodic function with a varying amplitude, which operation was used?"
Response:
After testing both addition and multiplication out, it seems as though both have varying amplitudes, but multiplication seems to be the most compelling.
EDIT I figured it out