15x+21y=39
30x+44y=81
х=13/5-7/5у
30х+44у=81
30(13/5-7/5у)+44у=81
у=3/2
х=13/5-7/5*3/2
х=1/2
(х,у)=(1/2,3/2)
15*1/2+21*3/2=39
30*1/2+44*3/2=81
39=39
81=81
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15x+21y=39
30x+44y=81
х=13/5-7/5у
30х+44у=81
30(13/5-7/5у)+44у=81
у=3/2
х=13/5-7/5*3/2
х=1/2
(х,у)=(1/2,3/2)
15*1/2+21*3/2=39
30*1/2+44*3/2=81
39=39
81=81
(х,у)=(1/2,3/2)