The degrees and radians!
% Sqr sqr ^ y ln log ^ y int sin cos tg ctg asin acos
x ^ 2 x ^ 3 x ^ y ^ xx * 2 10 x / January 10 / x 10 ^ xe ^ x pi ^ x sc csc
Linear, quadratic and biquadratic upavneniya! Fractions! Triangles! Vectors!
6 memory slots!
And it is very easy to use. One button is responsible for one action!
(Possible installation of "hot" keys, does not require installation)