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![](https://rs.olm.vn/images/avt/0.png?1311)
Cau 1:8*8*8*....*8*8=(8*8*8*8)*(8*8*8*8)*....*(8*8*8*8)*8
=(...6)*(....6)*(...6)*...(...6)*8=(...6)*8=....8
vay chu so tan cung la 8
Cau 2 121:(2+16+x/x)=11
2+16+x/x=121:11
18+x/x=11
x/x=18-11
x/x=7
![](https://rs.olm.vn/images/avt/0.png?1311)
a , tổng các phân số đã cho là : 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64 = 79/64
b, \(\frac{79}{64}\)và \(\frac{2017}{2018}\)= \(\frac{159422}{129152}\)và \(\frac{129088}{129152}\)= \(\frac{159422}{129152}\)> \(\frac{129088}{129152}\)
=> \(\frac{79}{64}\)> \(\frac{2017}{2018}\)
a) 1/2 + 1/4 + 1/8 + 1/ 16 + 1/32 + 1/64
=32/64 + 16/64 + 8/64 + 4/64 + 2/64
=32+16+8+4+2/64 = 66/64= 33/32
b) ta có 33/32 > 1 và 2017/2018<1
nên 33/32 > 2017/2018
![](https://rs.olm.vn/images/avt/0.png?1311)
a: 4A=4+4^2+...+4^9
=>3A=4^9-1
=>A=(4^9-1)/3
b: 2A=1+1/2+...+1/2^7
=>A=1-1/256=255/256
c: =1-1/5+1/5-1/9+...+1/85-1/89
=1-1/89=88/89
d: =1/3(3/1*4+3/4*7+...+3/304*307)
=1/3(1-1/4+1/4-1/7+...+1/304-1/307)
=1/3*306/307=102/307
e: E=1-1/2+1/2-1/3+...+1/11-1/12
=1-1/12=11/12
g: =2/5(1-1/6+1/6-1/11+...+1/96-1/101)
=2/5*100/101=40/101
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a)\) \(S=1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+...+\frac{1}{2187}\)
\(S=1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^7}\)
\(3S=3+1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^6}\)
\(3S-S=\left(3+1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^6}\right)-\left(1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^7}\right)\)
\(2S=3+\frac{1}{3^7}\)
\(2S=\frac{3^8+1}{3^7}\)
\(S=\frac{3^8+1}{3^7}.\frac{1}{2}\)
\(S=\frac{3^8+1}{2.3^7}\)
Vậy \(S=\frac{3^8+1}{2.3^7}\)
Chúc bạn học tốt ~
![](https://rs.olm.vn/images/avt/0.png?1311)
a) 128,5 : 0,5 + 128,5 x 3 + 128,5 : 0,25 + 128,5
=128,5 x 2 +128,5 x 3 +128,5 x 4 +128,5 x 1
=128,5 x ( 2 + 3 + 4 + 1 )
=128,5 x 10
=1285
b) 17,2 x 9,55 + 9,45 x 17,2 + 17,2
= 17,2 x 9,55 + 9,45 x 17,2 + 17,2 x 1
=17,2 x ( 9,55 +9,45 + 1 )
=17,2 x 20
=344
c) 6,45 : 0,2 + 6,45 : 0,25 + 6,45 - 6,45 : 0,1
=6,45 x 5 + 6,45 x 4 +6,45 x 1 + 6,45 x 10
=6,45 x (5 +4+1+10)
=6,45x20
=129
d) 1/2 : 0,5 - 1/4 : 0,25 + 1/8 : 0,125 - 1/10 : 0,1
=0,5;0,5-0,25:0,25+0,125:0,125-0,1:0,1
=1-1+1-1
=0