Bài 2:
a) `A= (2x+3y)^2 - (2x+3y)^2`
`A=(2x+3y -2x - 3y)(2x+ 3y + 2x + 3y)`
`A= 0. ( 4x + 6y)`
`A=0`
Vậy `A=0`
b) `B=(3x+1)^2 + (3x+1)(3x-1)`
`B= (3x+1)[( 3x+1) + (3x-1)]`
`B= (3x+1) ( 3x +1 + 3x-1)`
`B= (3x+1). 6x`
`B= 18x^2 + 6x`
Vậy `B= 18x^2 + 6x`
Bài 3:
`A= 16y^2 - 8y+1`
`A= (4y)^2 - 2. 4y . 1 + 1^2`
`A= (4y+1)^2`
Vậy `A= (4y+1)^2`
`B= (x+2)^2 - 2(x+2) .y + y^2`
`B = (x+2 -y )^2`
Vậy `B=(x+2-y)^2`
`C= -1/9 x^2 + 4/9 x - 4/9 + 49`
`C= -( 1/9 x^2 - 4/9 x + 4/9) + 49`
`C= - [(1/3 x )^2 - 2. 1/3 x . 2/3 + (2/3)^2] + 49`
`C= - ( 1/3 x - 2/3)^2 + 49`
`C= 7^2 - ( 1/3 x - 2/3)^2`
`C= ( 7- 1/3x + 2/3) ( 7 + 1/3 x - 2/3)`
`C= (23/3 - 1/3 x) ( 19/3 + 1/3 x)`
Vậy `C = (23/3 - 1/3 x) ( 19/3 + 1/3 x)`