Toán Lớp 8: Câu 1: ( 3 điểm ) Tính và rút gọn a) 3x^2 (2x^2-5x+4)-(6x^4-15x^3 ) b) -x^2 (-2x^2+3x-1)-(7x^4-5x^3 ) c) -2x^2 (3x-1/4-2x^3 )-(4x

Question

Toán Lớp 8:  Câu 1: ( 3 điểm )   Tính và rút gọn  a)  3x^2 (2x^2-5x+4)-(6x^4-15x^3 ) b)  -x^2 (-2x^2+3x-1)-(7x^4-5x^3 ) c)  -2x^2 (3x-1/4-2x^3 )-(4x^5-6x^3 ) d)(-3x+2)(3x-4)+(3x-1)(3x+1)  Câu 2: ( 2 điểm ) Tìm x biết  a)  (2x+1)(x-5)-x(3+2x)=26 b)  (7-4x)(4x+7)-8x(11-2x)=-39

dinhcongson · Answer

C&acirc;u 1:    a)3x&sup2;(2x&sup2;-5x+4)-(6x^4-15x&sup3;)   =6x^4-15x&sup3;+12x&sup2;-6x^4+15x&sup3;   =(6x^4-6x^4)+(-15x&sup3;+15x&sup3;)+12x&sup2;   =12x&sup2;   b)-x&sup2;(-2x&sup2;+3x-1)-(7x^4-5x&sup3;)   =2x^4-3x&sup3;+x&sup2;-7x^4+5x&sup3;   =(2x^4-7x^4)+(-3x&sup3;+5x&sup3;)+x&sup2;   =-5x^4+2x&sup3;+x&sup2;   c)-2x&sup2;(3x-1/4-2x&sup3;)-(4x^5-6x&sup3;)   =-6x&sup3;+1/2x&sup2;+4x^5-4x^5+6x&sup3;   =(-6x&sup3;+6x&sup3;)+1/2x&sup2;+(4x^5-4x^5)   =1/2x&sup2;   d)(-3x+2)(3x-4)+(3x-1)(3x+1)   =-9x&sup2;+12x+6x-8+9x&sup2;+3x-3x-1   =(-9x&sup2;+9x&sup2;)+(12x+6x+3x-3x)-(8+1)   =18x-9    C&acirc;u 2:    a)(2x+1)(x-5)-x(3+2x)=26   &hArr;2x&sup2;-10x+x-5-3x-2x&sup2;=26   &hArr;(2x&sup2;-2x&sup2;)+(-10x+x-3x)-5=26   &hArr;-12x-5=26   &hArr;-12x=26+5   &hArr;-12x=31   &hArr;x=-31/12   Vậy x=-31/12   b)(7-4x)(4x+7)-8x(11-2x)=-39   &hArr;(7-4x)(7+4x)-88x+16x&sup2;=-39   &hArr;7&sup2;-(4x)&sup2;-88x+16x&sup2;=-39   &hArr;49-16x&sup2;-88x+16x&sup2;=-39   &hArr;-88x+49=-39   &hArr;-88x=-39-49   &hArr;-88x=-88   &hArr;x=(-88):(-88)   &hArr;x=1   Vậy x=1

tongtung · Answer

Lời giải và giải thích chi tiết:    C&acirc;u 1:     a) 3x&sup2;.(2x&sup2;- 5x+4) - (6x 4 -15x&sup3; )     = 6x 4 - 15x&sup3; + 12x&sup2; - 6x 4 + 15x&sup3;     = (6x 4 - 6x 4 ) + ( -15x&sup3; +15x&sup3;)+ 12x&sup2;      = 12x&sup2;     b/. - x&sup2; (- 2x&sup2;+3x-1)- (7x 4 -5x&sup3;)     = 2x 4  - 3x&sup3; + x&sup2; - 7x 4  + 5x&sup3;     =  (2x 4 - 7x 4 ) + (5x&sup3; - 3x&sup3;) + x&sup2;    = - 5 x 4  +2 x&sup3; + x&sup2;   c/.  - 2x&sup2; (3x - 1/4 - 2x&sup3;)-(4x 5 - 6x&sup3;)     = - 6x&sup3; + 1/2x&sup2; + 4x 5  - 4x5 + 6x&sup3;     =  ( 4x 5  - 4x 5 ) + (- 6x&sup3; + 6x&sup3;) + 1/2x&sup2;      =  1/2x&sup2;     d/. (-3x+2)(3x-4) +(3x-1)(3x+1)     = - 9x&sup2; + 12x + 6x - 8 + [(3x)&sup2; - 1&sup2;]     = - 9x&sup2; + 12x + 6x - 8 + 9x&sup2; - 1     = (- 9x&sup2; + 9x&sup2;) + (12x + 6x) + (- 8 - 1)     = 18x - 9     C&acirc;u 2:     a) (2x+1)(x-5) - x(3+2x) = 26     &hArr; 2x&sup2; - 10x + x - 5 - 3x - 2x&sup2; - 26 = 0     &hArr; (2x&sup2; - 2x&sup2;) + (- 10x + x - 3x) + (- 5 - 26) = 0     &hArr; - 12x - 31 = 0     &hArr; - 12x = 31     &rArr; x = (31)/(-12)     &rArr; x = (-31)/(12)     Vậy S = { (-31)/(12)}     b) (7- 4x)(4x+7) - 8x(11-2x) =- 39     &hArr; 28x  + 49 - 16x&sup2; - 28x - 88x + 16x&sup2; + 39 = 0     &hArr; (- 16x&sup2;+ 16x&sup2;) + (- 28x - 88x +  28x  + (49 + 39) = 0     &hArr; - 88x + 88 =0      &hArr;  88x =  88      &rArr; x = (88)/(88)     &rArr; x = 1     Vậy S = { 1 }     Ch&uacute;c bạn học tốt nh&eacute;