a) 105*x-20-18-16-...-2=19+17+15+...+3+1

105*x=1+2+3+4+...+20

105*x=\(\frac{\left(20+1\right)x20}{2}\)

105*x=210

x=2

Vậy x = 2

b) 1-6+11-16+21-26+...+91-96+101

=101-96+91-86+...+21-16+11-6+1

=(101-96)+(91-86)+...+(21-16)+(11-6)+1

=5+5+...+5+5+1                ( có (101-6):5+1);2=10 số 5)

=5x10+1=51

a)105xX-20-18-16-...-2=19+17+15+...+3+1

105xX-(20+18+16+...+2)=19+17+15+...+3+1

105xX-110=100

105xX=100+110

105xX=210

X=210:105

X=2

a)14/13+(-1/13-19/20)=1/20 

b)-24/17-(-7/17-1/16)=-479/272

\(a,=\dfrac{14}{13}-\dfrac{1}{13}-\dfrac{19}{20}\\ =1-\dfrac{19}{20}=\dfrac{20}{20}-\dfrac{19}{20}\\ =\dfrac{1}{20}\\ b,=\dfrac{-24}{17}+\dfrac{7}{17}+\dfrac{1}{16}\\ =-1+\dfrac{1}{16}\\ =\dfrac{-16}{16}+\dfrac{1}{16}=\dfrac{-15}{16}\)

^{_{a)14/13+(-1/13-19/20)=1/20}}

^{_{b)-24/17-(-7/17-1/16)=-479/272}}

S = 1 - 2 + 3 + 4 - 5 - 6 + 7 + 8 - 9 + ... - 2006 + 2007 + 2008 - 2009

= 1 + (-2 + 3 + 4 - 5) + (-6 + 7 + 8 - 9) + ... + (-2006 + 2007 + 2008 - 2009)

= 1 + 0 + 0 + ... + 0

= 1

a, \(\dfrac{14}{13}-\dfrac{1}{13}-\dfrac{19}{20}=1-\dfrac{19}{20}=\dfrac{1}{20}\)

b, \(-\dfrac{24}{17}+\dfrac{7}{17}+\dfrac{1}{16}=\dfrac{-17}{17}+\dfrac{1}{16}=-1+\dfrac{1}{16}=-\dfrac{15}{16}\)

\(\dfrac{14}{13}+\left(\dfrac{-1}{13}-\dfrac{19}{20}\right)=\left(\dfrac{14}{13}+\dfrac{-1}{13}\right)-\dfrac{19}{20}=\\ \dfrac{13}{13}-\dfrac{19}{20}=1-\dfrac{19}{20}=\dfrac{20}{20}-\dfrac{19}{20}=\dfrac{1}{20}\)

Câu b em làm tương tự nhé

\(a.\dfrac{14}{13}+\left(\dfrac{-1}{13}+\dfrac{19}{20}\right)=\left(\dfrac{14}{13}+\dfrac{-1}{13}\right)-\dfrac{19}{20}=1+\dfrac{19}{20}=\dfrac{20}{20}-\dfrac{19}{20}=\dfrac{1}{20}\\ b.\dfrac{-24}{17}+\left(\dfrac{-7}{17}-\dfrac{1}{16}\right)=\left(\dfrac{-24}{17}+\dfrac{7}{17}\right)+\dfrac{1}{16}=-1+\dfrac{1}{16}=\dfrac{-16}{16}+\dfrac{1}{16}=\dfrac{-15}{16}\)

a. ( 4 x 25 ) x (5 x 9)

= 100 x 45

= 4500

b. (2008 x4) - ( 2008 x 4)

= 0

có một câu nữa : d)211/6 : (-4/5) - 271/6 : (-4/5)

a)13/2     b)9,925  c)-1  d)