Toán Lớp 8: Bài 1 : Tính : e. (x2 – 1)(x2 + 2x) f. (2x – 1)(3x + 2)(3 – x)

Question

Toán Lớp 8:  Bài 1 : Tính :  e. (x2 – 1)(x2 + 2x)                                                                     f. (2x – 1)(3x + 2)(3 – x)                        g. (x + 3)(x2 + 3x – 5)      h. (xy – 2).(x3 – 2x – 6)                    i. (5x3 – x2 + 2x – 3).(4x2 – x + 2)

diemchau · Answer

Giải đáp:   e) $ left(x^2-1 right) left(x^2+2x right)$   $=x^2x^2+x^2 cdot  :2x-1 cdot  :x^2-1 cdot  :2x$   $=x^4+2x^3-x^2-2x$   f) $ left(2x-1 right) left(3x+2 right) left(3-x right)$   $= left(6x^2+x-2 right) left(3-x right)$   $=6x^2(3&minus;x)+x(3&minus;x)&minus;2(3&minus;x)$   $=18x^2&minus;6x^3+x(3&minus;x)&minus;2(3&minus;x)$   $=18x^2&minus;6x^3+3x&minus;x^2&minus;2(3&minus;x)$   $=18x^2&minus;6x^3+3x&minus;x^2&minus;(6&minus;2x)$   $=18x^2&minus;6x^3+3x&minus;x^2&minus;6+2x$   $=(18x^2&minus;x^2)&minus;6x^3+(3x+2x)&minus;6$   $=17x^2&minus;6x^3+5x&minus;6$   g) $ left(x+3 right) left(x^2+3x-5 right)$   $=x(x^2+3x-5)+3(x^2+3x-5)$   $=x^3+3x^2-5x+3(x^2+3x-5)$   $=x^3+3x^2-5x+3x^2+9x-15$   $=x^3+(3x^2+3x^2)+(-5x+9x)-15$   $=x 3+6x^2+4x-15$   i) $ left(5x^3-x^2+2x-3 right) left(4x^2-x+2 right)$   $=5x^3(4x^2-x+2)-x^2(4x^2-x+2)+2x(4x^2-x+2)-3(4x^2-x+2)$   $=20x^5-5x^4+10x^3-x2(4x^2-x+2)+2x(4x^2-x+2)-3(4x^2-x+2)$   $=20x 5 -5x 4 +10x 3 -(4x 4-x ^3+2x^ 2)+2x(4x2-x+2)-3(4x2-x+2)$   $=20x^5-5x^4+10x ^3-(4x ^4-x ^3 +2x ^2)+8x ^3-2x ^2 +4x-3(4x ^2 -x+2)$   $=20x ^5 -5x ^4 +10x ^3 -(4x ^4 -x ^3 +2x ^2 )+ 8x ^3 -2x ^2 +4x-(12x ^2 -3x+6)$   $=20x ^5 -5x ^4 +10x ^3 -4x ^4 +x ^3 -2x ^2 + 8x ^3 -2x^ 2 +4x-12x ^2 +3x-6$   $=20x ^5 +(-5x ^4 -4x ^4 )+(10x ^3 +x ^3 + 8x ^3 )+(-2x ^2 -2x ^2 -12x ^2 )+(4x+3x)-6$   $=20x ^5 -9x ^4 +19x ^3-16x ^2+7x-6$