HI Askov,

Welcome to the forum.
addition (+) , multiplication (*) and division (/) available in Excel as separate function (operators)..are you looking all in one? I hope not.. the only thing here is that value of "a" can be 10 plus/minus 0.5 and value of "b" can be 20 plus / minus 0.7 and for this, you can use formula like Rand() with 10 or 20 to server plus / minus purpose.

Regards,
DILIPandey
<click on below * if this helps>