Ответ:

1.

[tex]+ \left \{ {{x+y=15} \atop {x-y=1}} \right. \\\\2x=16\\x=16/2\\x=8\\\\x-y=1\\8-y=1\\-y=1-8\\-y=-7\\y=7\\\left \{ {{y=7} \atop {x=8}} \right. \\[/tex]

2.

[tex]+ \left \{ {{x-y=9} \atop {x+y=7}} \right. \\\\2x=16\\x=16/2\\x=8\\\\x+y=7\\8+y=7\\y=7-8\\y=-1\\\\\left \{ {{y=-1} \atop {x=8}} \right.[/tex]