[ljlolel]

I found after learning Greek I could instantly read Cyrillic too

[triword]

Odd. According to this venn diagram, that would only give you 3 additional characters of Greek from what you would already know coming form English.

https://commons.wikimedia.org/wiki/File:Venn_diagram_showing...

[ipeev]

That diagram is rather bad at what it tries to do. Those are also historically and phonetically the same: Λ Л Δ Д Κ К The first Cyrillic alphabet was using the https://en.wikipedia.org/wiki/Glagolitic_script , curiously created by Saint Cyril, but then people found it was too difficult, so someone in the Preslav Literary School in the First Bulgarian Empire mashed up Glagolitic, Greek and Latin to create the new Cyrillic (probably naming it as a sorry to Cyril for butchering his nice unique alphabet).

[owenversteeg]

The diagram says that (Cyrillic ∩ Greek) - (Cyrillic ∩ Latin) is 3 letters, П Ф Г but as the sibling comment says, Λ/Л, Δ/Д and Κ/К are similar enough. That only leaves you with Θ/theta (th as in thin), Σ/sigma (s as in soft), Ξ/xi (x as in fox), Ψ/psi (ps as in lapse), and Ω/omega (o as in ore.) A lot of those are close enough that you can sort of guess, if you know the English names for the letters!

[cynicalkane]

Many Cyrillic letters are Latin-looking, but actually have direct Greek analogues due to the history of the writing system. If you don't know Greek letters, you'd have a hard time guessing р made a 'r' sound. If you do, it's a natural guess.

[Koshkin]

With the exception of some "ligatures" like Ю (I + O) and special characters like Ъ, Cyrillic is largely based on Greek and some Aramaic (e.g. Ш). In the past it included pretty much the entire Greek alphabet.

[huhtenberg]

And I found that after going through some math in the uni I could instantly read Greek !

The only letter that never saw any use in proofs was ι (iota).