Bài 5:
4: Ta có: \(5x\left(-x+1\right)-5\left(-x^2-x\right)=0\)
\(\Leftrightarrow-5x\left(x-1\right)+5x\left(x+1\right)=0\)
\(\Leftrightarrow5x\left(-x+1+x+1\right)=0\)
hay x=0
1) -3x ( -x + 3 )
= 3x2 - 9x
2) -5x3 ( -3x + 5)
= 15x4 -25x3
3) -2x ( -2x -6 )
= 4x2 + 12x
4) -3x5 ( -2x -12 )
= 6x6 + 36x5
5) -10x ( -5x + 2)
= 50x2 -20x
6) -2x2 ( -3x3 + 4x2 -5 )
= 6x5 - 8x4 + 10x2
7) -x2 ( -2x3 -x + 3 )
= 2x5 + x3 - 3x2
8) -2x3 ( -2x + x2 + 1)
= 4x4 - 2x5 - 2x3
9) ( -x -3 ) ( x+ 2)
= -x2 -2x -3x -6
= -x2 -5x -6
10 ) ( 2x - 3 ) ( 5 - x )
= 10x -2x2-15 + 3x
= -2x2 + 13x -15
11 ) ( -x + 6 ) ( -x -2 )
= x2 + 2x - 6x -12
= x2 -4x -12
12) ( 3x -1 ) ( -3 - 2x)
= -9x -6x2 + 3+ 2x
= -6x2 - 7x + 3
13) ( -5x -3)(2 - x )
= -10x + 5x2 - 6 + 3x
= 5x2 -7x -6
14) ( 2x -1 ) ( 4x2 +2x +1 )
= 8x3 - 1
15) ( x- 3 ) ( 1 - 2x - 5y )
= x -2x2 -5xy -3 + 6x + 15y
= -2x2 + 7x - 5xy + 15 y -3
16) ( x-2 ) ( x2+4 ) ( x + 2 )
= (x2 -4) ( x2 + 4 )
= x4 - 16
17) (-3x + 1) (9x2 -1 ) ( -3x -1 )
= ( 9x2 - 1 )( 9x2 -1 )
= 81x4 - 18x2 + 1
18 ) - (2x + 1) ( x- 1 ) ( x+ 1 )
= -( 2x + 1 ) ( x2 -1 )
= - ( 2x3 - 2x + x2 -1 )
= - 2x3 + 2x -x2 + 1
19 ) ( 2x2 - 3x + 5 )( x2 - 8x + 2 )
= 2x4 - 16x3 + 4x2 - 3x3 + 24x2 -6x + 5x2 - 40x + 10
= 2x4 -19x3 + 33x2 -40x +10
20) ( 3x2y - 6xy + 9x ) ( - xy )
= - 3x3y2 + 6x2y2 - 9x2y
