5a-10b=18;⇒5(a-2b)=18;
(a-2b)=18/5=3.6;
(a²-4b²)/(0.9a+1.8b)=(a-2b)(a+2b)/0.9·(a+2b)=(a-2b)/0.9=3.6/0.9=4;
(m+12)*(y-3)
=
my - 3m + 12y - 36
4cosx - 4sinx - 4sin2x = 0
4cosx - 4sinx - 8sinxcosx = 0 | : 8cosx
2 - 4tgx - tgx = 0
2 - 5tgx = 0
-5tgx = -2
tgx = 2/5
x = arctg2/5 + Pi*n, n € Z