Toán Lớp 8: Rút gọn: b/ ( 2x – 3 )^2 – ( x – 3 ) ( x^2 – 5x + 4 )
c/ ( x + 8 ) ( 2x – 4 ) – ( x – 5 )^2
e/ ( 3x + 1 )^2 – 3x ( x + 1 ) ( x – 2 )
f/ ( 3x – 4 ) ( 2 + 3x ) – ( 3x + 5 )^2
i/ ( 2 – 9x ) ( 2 – x ) – ( 3x + 2 )^2
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= 4x^2−12x+9+(−x+3)(x^2−5x+4)
= 4x^2−12x+9−x^3+8x^2−19x+12
= 12x^2−31x+21−x^3
= −x^3+12x^2−31x+21
c)( x + 8 ) ( 2x – 4 ) – ( x – 5 )^2
= 2x^2+12x−32−x^2+10x−25
= (2−1)x^2+(12+10)x+(−32−25)
= x^2+22x−57
e)( 3x + 1 )^2 – 3x ( x + 1 ) ( x – 2 )
= 9x^2+6x+1−3x^3+3x^2+6x
= 12x^2+12x+1−3x^3
= −3x^3+12x^2+12x+1
f)( 3x – 4 ) ( 2 + 3x ) – ( 3x + 5 )^2
= 9x^2+18x+8−9x^2−30x−25
= −12x−17
i)( 2 – 9x ) ( 2 – x ) – ( 3x + 2 )^2
= 9x^2−20x+4−9x^2−12x−4
= −32x+(4−4)
= −32x
= −3x^3+12x^2+12x+1