Môn Toán Lớp 8: Rút gọn biểu thức: a) (2x=5)^3-30x(2x+5)-2x^3-8x^3 ;b) ( x+4)(x-4)-2x(x-3)+(x+3)^2 ; c) (x-1)^3-(x+1)^3-6(x-1)(x+1)1

Trả Lời

1. a)

   (2x-5)^3 – 30x (2x+5) – 2x^3 – 8x^3

   = 8x^3 – 60x^2 + 150x -125 – 60x^2 – 150x – 2x^3 – 8x^3

   = (8x^3-8x^3-2x^3) – (60x^2 + 60x^2) + (150x-150x) – 125

   =  -2x^3 – 120x^2 – 125

   b)

   (x+4)(x-4) – 2x (x-3) + (x+3)^2

   = (x^2 – 4^2) – 2x^2 + 6x + (x^2+6x+9)

   = x^2 – 16 – 2x^2 + 6x + x^2+6x+9

   = (x^2+x^2-2x^2) + (6x+6x) + (9-16)

   = 12x – 7

   c)

   (x-1)^3 – (x+1)^3 – 6 (x-1)(x+1)

   = (x^3 – 3x^2 + 3x – 1) – (x^3+3x^2+3x+1) – 6 (x^2-1)

   = x^3 – 3x^2 + 3x – 1 – x^3 – 3x^2-3x-1-6x^2+6

   = (x^3-x^3) – (6x^2+3x^2+3x^2) + (3x – 3x) + (6-1-1)

   = – 12x^2 + 4

   Trả lời