mấy cái này bấm máy tính được mà

13 ) (12x - 5)(4x - 1) + (3x - 7)(1 - 16x) = 81
<=> 48x^2 - 20x - 12x + 5 + 3x - 7 - 48x^2 + 112x = 81
<=> ( 48x^2 - 48x^2 ) + ( 112x - 20x - 12x + 3x ) + 5 - 7 = 81
<=> 83x -2 = 81
<=> 83x = 83
<=> x = 1
Vậy x = 1
15 ) ( 1/2x + y ) (1/2x + y )
= 1/4x^2 + 1/2xy + 1/2xy + y^2
= 1/4x^2 + xy + y^2
( x - 1/2y ) ( x - 1/2y )
= x^2 - 1/2xy - 1/2xy + 1/4y^2
= x^2 - xy + 1/4y^2

bai 15
a)
(1/2x+y)(1/2x+y) =(1/2x+y)^2 =1/4x^2 +xy +y^2
b)
( x-1/2y)( x-1/2y) =( x-1/2y)^2 = x^2 -xy +y^2 /4

13 ) (12x - 5)(4x - 1) + (3x - 7)(1 - 16x) = 81
<=> 48x^2 - 20x - 12x + 5 + 3x - 7 - 48x^2 + 112x = 81
<=> ( 48x^2 - 48x^2 ) + ( 112x - 20x - 12x + 3x ) + 5 - 7 = 81
<=> 83x -2 = 81
<=> 83x = 83
<=> x = 1
15 ) ( 1/2x + y ) (1/2x + y )
= 1/4x^2 + 1/2xy + 1/2xy + y^2
= 1/4x^2 + xy + y^2
( x - 1/2y ) ( x - 1/2y )
= x^2 - 1/2xy - 1/2xy + 1/4y^2
= x^2 - xy + 1/4y^2

7 ) ( x^2 - 2x + 1) ( x - 1 )
= ( x- 1)^2 ( x- 1)
= ( x-1)^3

( x^3 - 2x^2 + x - 1 ) ( 5 - x )
= 5x^3 - 10x^2 + 5x - 5 - x^4 + 2x^3 - x^2 + x
= -x^4 + 7x^3 - 11x^2 + 6x - 5
=> (x^3 - 2x^2 + x - 1 ) ( x - 5 )
= ( x^3 - 2x^2 + x - 1 ) . ( 5 - x ) . -1
= ( -x^4 + 7x^3 - 11x^2 + 6x - 5 ) .-1
= x^4 - 7x^3 + 11x^2 - 6x + 5