Mathematical Rebus II

Question

Mathematical Rebus I

Mathematical Rebus III

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$$\det\frac{\partial(x,y)}{\partial(r,\varphi)}\\ \sum_{n=2}^\infty \ddot{\frac{t^n}{n!}} $$

Answer 1

Follow up to rand al'thor answer

The graph shows:

sec(x)

By combining the 2 other answers, we obtain:

sec + r + et = $secret$

Answer 2

Graph:

work in progress...

$\det\frac{\partial(x,y)}{\partial(r,\varphi)}=r$, because

$x,y$ are Cartesian coordinates and $r,\varphi$ are polar coordinates, in the 2-dimensional plane.

$\sum_{n=3}^\infty \ddot{\frac{t^n}{n!}}=e^t$, because

differentiate $t^n$ twice to get $n(n-1)t^{n-2}$, so the sum is $\sum_{n=3}^\infty \ddot{\frac{t^{n-2}}{(n-2)!}} = \sum_{n=1}^\infty \ddot{\frac{t^n}{n!}}=e^t$.