~ Bạn tham khảo ~
`a, (2x+5y)(2x-5y)-4x^2`
`= (2x)^2-(5y)^2-4x^2`
`= 4x^2-25y^2-4x^2`
`=-25y^2`
`b, (1/2a^2+3b)^2-3a^2b-1/4a^4`
`= (1/2a^2)^2+2*1/2*3a^2b+(3b)^2-3a^2b-1/4a^4`
`= 1/4a^4+3a^2b+9b^2-3a^2b-1/4a^4`
`= 9b^2`
`c, (3x+5y)(9x^2-15xy+25y^2)-125y^3`
`= 3x(9x^2-15xy+25y^2)+5y(9x^2-15xy+25y^2)-125y^3`
`= 27x^3-45x^2y+75xy^2+45x^2y-75xy^2+125y^3-125y^3`
`=27x^3`
