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4 tháng 2 2016

-210 = (-1) + (-2) + (-3) + ... + (-x + 1) + (-x)

=> -210 = -(1 + 2 + 3 + ...+ x - 1 + x)

=> 210 = 1 + 2 + 3 + x - 1 + x

=> 210 = \(\frac{\left(x+1\right).x}{2}\)

=> 420 = (x + 1).x

=> 21 . 20 = (x + 1) . x

=> (20 + 1) . 20 = (x + 1) . x

=> x = 20

25 tháng 1 2017
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3 tháng 2 2016

a) (3x -2)^5 = -243 
(3x -2)^5 = (-3)^5 
3x-2=-3 
3x=-1 
x=-1/3 
b) ( x-7)^( x+1) - ( x-7)^(x+11) = 0 
( x-7)^( x+1) - ( x-7)^(x+1)*x^10 = 0 
( x-7)^( x+1) (1-x^10) = 0 
Do x phải là số nguyên tố nên x=7

20 tháng 1 2017

sai rùi

12 tháng 2 2016

bai toan nay khó

12 tháng 2 2016

bài này khó

12 tháng 2 2016

x= 20

duyệt nhé

-x=-20

12 tháng 2 2016

x = 20 ; -x = -20, ung  hộ  mk nha bn

14 tháng 2 2016

Ta có: -210=(-1)+(-2)+(-3)+...+(-x+1)+(-x)

          -210=-[1+2+3+...+(x-1)+x)]

          -210=\(-\left[\frac{x\left(x+1\right)}{2}\right]\)

     hay 210=\(\left[\frac{x\left(x+1\right)}{2}\right]\)

     => x(x+1)=210.2=420

     => 20.21=420 => x=20

16 tháng 2 2016

x=20 thi thoa man de bai

25 tháng 1 2017

20 là đúng

15 tháng 2 2016

X=-20

Cần cách lm ko?

15 tháng 2 2016

x= -20

tich ủng hộ nhé