Mk viết bằng 2 cách nhé!

Cách 1 : 

A = {8 ; 9 ; 10 ; ... ; 97 ; 98 ; 99}

Cách 2 : 

A = {x\(\in\)N* | 7 < x < 100}

~Study well~

#KSJ

Để 20ab chia hết cho 9

=> 2 + 0 + a + b chia hết cho 9

=> 2+ a + b chia hết cho 9

=> a + b = 9 - 2 = 7 

=> b = 9 , 8 ,7 

=> a = 2 , 1 , 0 

( a, b nhỏ hơn 10)

20ab \(⋮\)9

=> a + b = 7

=> 

\(\frac{17}{21}\cdot\frac{48}{53}+\frac{17}{21}\cdot\frac{4}{53}+\frac{52}{53}\cdot\frac{4}{21}\)

\(=\frac{17}{21}\left(\frac{48}{53}+\frac{4}{53}\right)+\frac{52}{53}\cdot\frac{4}{21}\)

\(=\frac{17}{21}\cdot\frac{52}{53}+\frac{52}{53}\cdot\frac{4}{21}\)

\(=\frac{52}{53}\left(\frac{17}{21}+\frac{4}{21}\right)\)

\(=\frac{52}{53}\cdot1\)

\(\frac{17}{21}.\frac{48}{53}+\frac{17}{21}.\frac{4}{53}+\frac{52}{53}.\frac{4}{21}\)

= \(\frac{17}{21}.\left(\frac{48}{53}+\frac{4}{53}\right)+\frac{52}{53}.\frac{4}{21}\)

= \(\frac{17}{21}.\frac{52}{53}+\frac{52}{53}.\frac{4}{21}\)

=\(\frac{52}{53}.\left(\frac{17}{21}+\frac{4}{21}\right)\)

= \(\frac{52}{53}\)

a) 

\(A=2+2^2+2^3+2^4+...+2^{19}+2^{20}\)

\(A=\left(2+2^2\right)+\left(2^3+2^4\right)+...+\left(2^{19}+2^{20}\right)\)

\(A=\left(2+2^2\right)+2^2\left(2+2^2\right)+...+2^{18}\left(2+2^2\right)\)

\(A=6+2^2\cdot6+...+2^{18}\cdot6\)

\(A=3\cdot2+2^2\cdot3\cdot2+...+2^{18}\cdot3\cdot2\)

\(A=3\left(2\cdot2^3+...+2^{19}\right)⋮3\) (đpcm)

Còn phần b) tớ chịu =))

Đề là?

Đặt \(A=\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{100}}\)

\(\Rightarrow3A=1+\frac{1}{3}+...+\frac{1}{3^{99}}\)

\(\Rightarrow3A-A=\left(1+...+\frac{1}{3^{99}}\right)-\left(\frac{1}{3}+...+\frac{1}{3^{100}}\right)\)

\(\Rightarrow2A=1-\frac{1}{3^{100}}\)

\(\Rightarrow A=\frac{1-\frac{1}{3^{100}}}{2}\)

a)\(\left(-4\right)+\left(-29\right)+\left(-15\right)+29\)

\(=-19\)

cac ban oi vao lam ho mk bai mk k cho ban y nhe

a) (5/12+3/8).(-3/7+5/14)

     =(10/24+9/24).(-6/14+5/14)

     =19/24.-1/14

     =-19/336

b)106/35