I am trying to understand what these stickers mean.
The ones on the top row are:
Good one!
I am impressed!
(I know $\sum x_n /n$ is the mean of the $x_i$, but I don't understand this one)
The bottom row is:
This improvement is a good sign
(I know $\sin(\pi/2-\theta)=\cos\theta$, but I also don't get this one)
Awesome work!
3: "This work is above average" ($\sum{x}_n/n$ is indeed the average)
5: "Complementary - this is good work" (cosine is the complementary angle of sine)