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I'm trying to understand how the sandwich/ squeeze theorem establishes that $lim_thetato 0sintheta = 0$ and $lim_thetato 0costheta = 1$.

Mainly, i'm trying to understand this logic, rather than trying to answer an assignment question.

The idea behind sandwich/squeeze theorem is intuitive enough. If values of a function $f$ are lying (sandwiched) between those of $g, h$ and values of $g, h$ tend to a common limit then that of $f$ also tend to the same limit.

We have the fundamental inequality $|sin x|leq |x|$ for $0<|x|<pi/2$ which is the same as $$-|x|leq sin xleq |x|$$ for $0<|x|<pi/2$ and since both $-|x|$ and $|x|$ tend to $0$ as $xto 0$ it follows that $lim_xto 0sin x=0$.

For $cos x$ the situation is tricky and we need to use a bit of trigonometry. We have $$cos x=1-2sin^2(x/2)$$ and noting the earlier inequality for $sin $ we get $$1-fracx^22leq cos xleq 1$$ And by squeeze we get $lim_xto 0cos x=1$.

Edit: I corrected some inaccuracies pointed out in the comments.

I try to explain the logic by solving the exercize. First of all, this result is also known as the two policemen theorem because if two policemen are escorting a person between them, and both officers approch arbitrary close to the cell, then the prisoner must also approch arbitrary close to it (notice that the policemen don't need to go to the cell; this means the functions need not to be defined in the limit point). This summarizes the main idea of the theorem.

Now, we know that when $x$ is sufficiently near to $0$ (i.e. the functions must be defined in a neighbourhood of such a point), $-lvert xrvertle sin xle lvert xrvert$ and this holds for any $x$ sufficiently small (actually for any $lvert xrvert $ in $]0,pi/2[$). Since limits preserve the linear order, we have that if limit of $sin x$ exists, then $$lim_xrightarrow0 -xle lim_xrightarrow0 sin x le lim_xrightarrow0 x.$$
But the first and the last limits are $0$, hence limit of $sin x$ exists and is $0$ because this function is eventually bounded both from above and below by functions converging to the same limit, so $sin x$ can't go away.

Generally speaking, you need two functions/policemen who bound the function of which you want to compute the limit, from above and below respectively, in a neighbourhood of the limit point and they must have the same limit at that point.