Why in the mathemathic classes everythig must be perfect? ?

Because one small error can give you an entirely different answer than the correct one . for example ;

10= 2x + 2
let's say instead subtracting on both sides first you divide 2 on both sides and get this ; 6= x , which is the incorrect answer. The correct answer would've been 4 = x if you had subtracted first . You got in incorrect because you mixed up one step of your order of operations. So you see, every step you do in math , even the order is critical in finding the correct solution .