(3х+у+9)(5х+у-5)=
=3х*5х+3х*у-3х*5+у*5х+у*у-у*5+9*5х+9*у-9*5=
=15х(2)+3ху-15х+5ху+у(3)=
=15х(2)+8ху-15х+у(3)
(3х+у+9)(5х+у-5)=
=3х*5х+3х*у-3х*5+у*5х+у*у-у*5+9*5х+9*у-9*5=
=15х(2)+3ху-15х+5ху+у(3)=
=15х(2)+8ху-15х+у(3)