`2)7^2-2.7.4y+(4y)^2`
`=(7-4y)^2`
`=(7-4.5)^2`
`=(-13)^2`
`=169`
`4)(7x)^2-(4y)^2`
`=(7x-4y)(7x+4y)`
`=(7.1-4.-5)(7.1+4.-5)`
`=(7+20)(7-20)`
`=27.-13`
`=-351`
`6)(4x)^3+3.5^2-3.(2x)(3x)^2`
`=(4.-1)^3+3.25-3.(2.-1)(3.-1)^2`
`=-64+75+54`
`=65`
`8)7^2-4.5.(y^2-2y)+20y^2` (sửa đề)
`=49-20y^2+40y+20y^2`
`=49+40y`
`=49+40.5`
`=249`