1)= 2x³ +6x²+ 8x -5x² -15x +20 +15x²+6x-20x-8
= 2x³+ 16x²-21x-28
2)= 2x³- 2x²+x +6x² -6x +3- x³+6x +4x²-24
= x³ +8x² +x-21
3)= ( x² +5x -4x -20 ) × (x-6) -x² -7x
= x³ -6x² + x² -6x -20x +120 -x² -7x
= x³ -6x² -33x +120
4)= x³ -9x² +27x -27 +x² -2x +1 +x² -2x -2x
= x³ -7x² +21x -26
5)= x³ +3x² -2x² -6x +x +3 +x² +2x -6x -12-3
= x³ +2x² -9x -12
