(а)12х+5у)/20х²у-(5у-4х)/25ху²=
(5у(12х+5у)-(5у-4х)*4х)/100х²у²=
(60ху+25у²-20ху+16х²)/(10ху)²=
(16х²+40ху+25у²)/(10ху)²=((4х+5у/(10ху)²
б)(2n+3m)/12mn²-(9m-2n)/18m²n=
((2n+3m)*3m-(9m-2n)*2n)/36m²n²=
=(6mn+9m²-18mn+4n²)/(6mn)²=
(9m²-12mn+4n²)/(6mn)²=
((3m-2n)/(6mn))²