7х-9у=11 -9у=11-7х у=(7/9)х-(11/9)=(7/9)х-1 2/9
15х-12у=0 12у=15х у=(15/12)х=1,25х
2х+3у=57 3у=57-2х у=19-(2/3)х
Tg2x=-(√3/3)
2x=-arctg(√3/3)+πn ,n∈Z
2x=-arctg(1/√3)+πn, n∈Z
2x=-π/6+πn, n∈Z
x=-π/12+(πn)/2 , n∈Z
3*25=3*5^2х-4
5^2х-4=5^2
2х-4=2
2х=6
х=3