Вроде так :
2а-а=а
-15+6=-9
а-9
2а-15-а+6=а-9
1) 3y2x(3+y)=6xy(3+y)+18xy+6xy^2
2)2y(y+2)=2y^2+4y
Cos 20 - sin50 = sin 70-sin50 = 2.sin(70-50)/2 . cos (70+50)/2 =
=2.sin10 . cos60= 2. sin10 . 1/2 = 2.(1/2).sin10 = sin10
<span>(sin x - sin y =2. sin(x-y)/2.cos (x+y)/2 )</span>
1. (b²/(a³-ab²)+1/(a+b)=b²/(a*(a²-b²))+1/(a+b)=b²/(a*(a-b)(a+b))+1/(a+b)=
=(b²+a*(a-b))/(a*(a+b)(a-b))=(a²+b²-ab)/(a*(a+b)(a-b)).
2. (a-b)/(a²+ab)-a/(b²+ab)=(a-b)/(a*(a+b))-a/(b*(a+b))=
=((a-b)*b-a*a)/(ab*(a+b))=(ab-b²-a²)/(ab*(a+b))=-(a²+b²-ab)/(ab*(a+b)).
3. (a²+b²-ab)/(a*(a+b)(a-b)):(-(a²+b²-ab)/(ab*(a+b))=-b/(a-b)=b/(b-a).