For (1), we started with the Maclaurin series 1/x to get us familiar with the idea of differential expansions, and then we moved to Taylor to derive expansions of some common functions like cos and sin:

cos(x) = 1 - x²/2! + x⁴/4! - …

sin(x) = x - x³/3! + x⁵/5! - …

We now start with the definition of e^x Taylor expansion, and proceed to do some substitutions:

e^x = 1 + x + x²/2! + x³/3! + … + xⁿ/n!

We can then substitute in: x=iθ (remembering that i² = -1) to get

e^iθ = 1 + iθ - θ²/2! - iθ³/3! + θ⁴/4! + iθ⁵/5! + … etc…

If we group by real and complex, we can arrange the above as:

e^iθ = (1 - θ²/2! + θ⁴/4! + … ) + i(θ - θ³/3! + θ⁵/5! + … )

You should now realise that the left part resembles the expansion of cos(θ), and the right part resembles sin(θ). That is:

e^iθ = cos(θ) + i sin(θ)

Finally, we substitute in θ = π

e^iπ = cos(π) + i sin(π)

And we know that cos(π) = -1, and that sin(π) = 0, meaning that we end up with

e^iπ = -1 + i 0

or

e^iπ + 1 = 0

The teacher got excited because it is literally one of the most beautiful mathematical statements you can get, that connects five universal identities under a single statement: 0, 1, e, i, and π – and does so using 3 different operators (times, power, plus).

For (2), I’m still waiting as I think it’s currently holding the world together by sheer mass alone