4.
a.
`(2x+1)^2-4(x+2)^2=9`
`⇔4x^2+4x+1-4(x^2+4x+4)=9`
`⇔4x^2+4x+1-4x^2-16x-16=9`
`⇔(4x^2-4x^2)+(4x-16x)=9+16-1`
`⇔-12x=24`
`⇔x=-2`
Vậy phương trình có nghiệm `x=-2`
c.
`3(x+2)^2+(2x-1)^2-7(x+3)(x-3)=36`
`⇔3(x^2+4x+4)+(4x^2-4x+1)-(7x+21)(x-3)=36`
`⇔3x^2+12x+12+4x^2-4x+1-7x^2+21x-21x+63=36`
`⇔(3x^2+4x^2-7x^2)+(12x-4x+21x-21x)=36-12-1-63`
`⇔8x=-40`
`⇔x=-5`
Vậy phương trình có nghiệm `x=-5`
5.
b.
`29.31`
`=(30-1)(30+1)`
`=30^2-1^2`
`=900-1`
`=899`
`39.41`
`=(40-1)(40+1)`
`=40^2-1^2`
`=1600-1`
`=1599`
c.
`56^2-46^2`
`=(56-46)(56+46)`
`=10.102`
`=1020`
`67^2-56^2`
`=(67-56)(67+56)`
`=11.123`
`=1353`