Ответ:

  1. \(4.1x(0.4x+0.5y)=1.64x^2+2.05xy\).
  2. \((2x-7y)(4x+3y)=8x^2+6xy-28xy-21y^2=8x^2-22xy-21y^2\).
  3. \((5x-3y)^2=25x^2-30xy+9y^2\).

Ответ: а) \(1.64x^2+2.05xy\); б) \(8x^2-22xy-21y^2\); в) \(25x^2-30xy+9y^2\).