Both sides evaluate to n(n+1)(2n+1)/2 using only 1) counting instances of n², and 1) using 1+2+3+···+n=n(n+1)/2.
I’m sure there are easier ways.

But non-Euclidean are great! After all, Foucault’s pendulum is just parallel translation on a sphere, which is the same as parallel translation on the cone tangent to the sphere on that line of latitude. And that cone practically Euclidean! Super interesting! Don’t tap out!

That’s a beautiful photo.
It makes me realize that I’ve changed the light transition lines to the midline in my photos to act better as phone wallpaper!

Yes, ditch that textbook!