Bài `3.` 

`a)`

`4x^2+4xy+y^2`

`=(2x)^2+2.2x.y+y^2`

`=(2x+y)^2`

Vậy `4x^2+4xy+y^2=(2x+y)^2`

`b)` 

`9m^2+n^2-6mn`

`=(3m)^2-2.3m.n+n^2`

`=(3m-n)^2`

Vậy `9m^2+n^2-6mn=(3m-n)^2`

`c)` 

`16a^2+25b^2+40ab`

`=(4a)^2+2.4a.5b+(5b)^2`

`=(4a+5b)^2`

Vậy `16a^2+25b^2+40ab=(4a+5b)^2`

`c)`

`x^2+x+1/4`

`=x^2+2.x.1/2+(1/2)^2`

`=(x+1/2)^2`

Vậy `x^2+x+1/4=(x+1/2)^2`

``

Bài `4.`
`a)`

`1001^2`

`=(1000+1)^2`

`=1000^2+2.1000.1+1^2`

`=1000000+2000+1`

`=1002001`

`b)` 

`29,9.30,1`

`=(30-0,1).(30+0,1)`

`=30^2-0,1^2`

`=900-0,01`

`=899,99`

`c)` 

`1011,24-63,6.21,8+475,24`

`=31,8^2-2.31,8.21,8+21,8^2`

`=(31,8-21,8)^2`

`=10^2`

`=100`