A=7/3(1/10-1/11+1/11-1/12+......+1/69-1/70

A=7/3(1/10-1/70)

A=7/3.3/35

A=1/5

tick cho mk nha

đây là cách tinh nhanh bn nha

=(2^3.9^4+9^4.5):(9^2.9)

=9^4(2^3+5):9^3

=9.13=117

tick cho mk nha

\(\dfrac{1.2}{3.2}+\dfrac{1.2}{6.2}+.....+\dfrac{1.2}{2\left(x+1\right):2.2}\)=\(\dfrac{1998}{2000}\)

\(\dfrac{2}{6}+\dfrac{2}{12}+.....+\dfrac{2}{x\left(x+1\right)}\)=\(\dfrac{1998}{2000}\)

\(2\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+.....+\dfrac{1}{x}-\dfrac{1}{x+1}\right)=\dfrac{1998}{2000}\)

\(2\left(\dfrac{1}{2}-\dfrac{1}{x+1}\right)=\dfrac{1998}{2000}\)

\(\dfrac{1}{2}-\dfrac{1}{x+1}=\dfrac{1998}{2000}:2\)

\(\dfrac{1}{x+1}=\dfrac{1}{2}-\dfrac{999}{2000}\)

\(\dfrac{1}{x+1}=\dfrac{1}{2000}\)

suy ra x+1=2000

suy ra x=2000-1=1999

tick cho mk nha

A=(3+3^2+3^3+3^4)+......+(3^1995+3^1996+3^1997+3^1998)

A=3(1+3+3^2+3^3)+.......+3^1995(1+3+3^2+3^3)

A=3.40+......+3^1995.40

A=120+.....+120.3^1994

A=120.(1+...3^1994) chia hết cho 120

tick cho mk nha

c,vì 8 chia hết cho 8 suy ra 2.6.8.10 chia hết cho 8

mà 40 cũng chia hết cho 8

suy ra A chia hết cho 8