a/ 1+2+3+4+……+ n
b/ 2+4+6+8+……+ 2.n
c/ 1+3+5+…..+ ( 2.n + 1)
d/ 1+4+7+10+…….+2005
e/ 2+5+8+……+2006
g/ 1+5+9+…….+2001
a/ 1+2+3+4+……+ n b/ 2+4+6+8+……+ 2.n c/ 1+3+5+…..+ ( 2.n + 1) d/ 1+4+7+10+…….+2005 e/ 2+5+8+……+2006 g/ 1+5+9+…….+2001
By Kaylee
By Kaylee
a/ 1+2+3+4+……+ n
b/ 2+4+6+8+……+ 2.n
c/ 1+3+5+…..+ ( 2.n + 1)
d/ 1+4+7+10+…….+2005
e/ 2+5+8+……+2006
g/ 1+5+9+…….+2001
Đáp án:a)$=\dfrac{1}{2}n.(n+1)$
b)=$n(n+1)$
c)$=(n+1)^2$
d)$=1003.669$
e)$=1004.669$
g) $=1001.501$
Giải thích các bước giải:
a)$1+2+..+n$
$=\dfrac{1}{2}n.(n+1)$
b)$2+4+6+..+2n$
=$2(1+2+3+..+n)$
=$2.\dfrac{1}{2}n(n+1)$
=$n(n+1)$
c)$1+3+5+..+(2n+1)$
$=\dfrac{1}{2}(2n+1+1)(n+1)$
$=(n+1)^2$
d$)1+4+7+…+2005$
$=\dfrac{1}{2}(2005+1)(\dfrac{2005-1}{3} +1)$
$=\dfrac{1}{2}.2006.669$
$=1003.669$
e)$2+5+8+..+2006$
$=\dfrac{1}{2}(2006+2)(\dfrac{2006-2}{3} +1)$
$=\dfrac{1}{2}.2008.669$
$=1004.669$
g)$1+5+9+…+2001$
$=\dfrac{1}{2}(2001+1)(\dfrac{2001-1}{4} +1)$
$=\dfrac{1}{2}.2002.501$
$=1001.501$