TỠM X BIẾT A) X + 2X + 3X + 4X + …..+ 2011X = 2012.2013X X X X B)...
Bài 1: Tỡm x biết
a) x + 2x + 3x + 4x + …..+ 2011x = 2012.2013
x
x
x
x
b)
1
2
3
4
2011 2010 2009
2008
HD : a) x + 2x + 3x + 4x + …..+ 2011x = 2012.2013
x( 1 + 2 + 3 + ….+ 2011) = 2012.2013
.
2011.2012
2012.2013
2.2013
x
2
x
2011
b) Nhận xột : 2012 = 2011+1= 2010 +2 = 2009 +3 = 2008 +4
Từ
1
2
3
4
(
2012) 2011 (
2012) 2010 (
2012) 2009
(
2012) 2008
x
x
x
x
2011
2010
2009
2008
x
x
x
x
2012
2012
2012
2012
2011
2010
2009
2008
2
1
1
1
1
(
2012)(
)
2
x
2011 2010 2009 2008
2 : (
) 2012