Question

Can someone explain me why we take square of [yi-(axi+b)] and then how we simplify the 2 from the deviation of that equation -least square session 29-

Answer 1

We taking the square cause we need absolute value of distance.
I don't understand your second question. If it's about taking deriviative try to look "Single variable calculus". If about simplifying just open brackets.

Answer 2

thank you, I meant the second one,simplifying, I got it

Answer 3

we can remove the 2 because we are setting the partials equal to 0. Therefore we can divide both sides by 2.