1)=2х-3+2х^2-х+81=х+2х^2+78
2)=14аб-9а^2-3б^2+3а^2-5аб+4б^2=9аб-6а^2+б^2
3)=2а^3-6б^3-(2а^3-6аб-5а^3+3аб+ +7б^3)=2а^3-6б^3-2а^3+6аб+5а^3-3аб-7б^3=5а^3-13б^3+3аб
2.
=7/12х^2+2/9ху-1/2х^2+1/3ху- 1/12х^2-5/9ху=
=(7/12х^2-6/12х^2-1/12х^2)+ (2/9ху+3/9ху-5/9ху)=0
3.
12-3х^2-5х-8х+3х^2=0
12-13х=0
13х=12
х=12/13