When I was in high school and saw the following equation, my mind was blown!

$$1+\frac{1}{4}+\frac{1}{9}+\frac{1}{16}+…=\frac{\pi^2}{6}$$

Why is Pi here? Isn’t it supposed to show up only in circle related problem? But the left-hand has nothing to do with circle. And it’s Pi squared!

Even today I still do not have an intuitive understanding why the above equation is true, until I watched a visual explanation from 3blue1brown. This video provide an elegant and visual proof of the equation. Go ahead and watch it:

The link is:

https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw/featured