\(A=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+....+\frac{1}{3^8}\)

\(=>3A=1+\frac{1}{3}+\frac{1}{3^2}+.....+\frac{1}{3^7}\)

\(=>3A-A=\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^7}\right)-\left(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^8}\right)\)

\(=>2A=1-\frac{1}{3^8}=>A=\left(1-\frac{1}{3^8}\right):2\)

Việtko đọc lưu ý à

BCNN(2;3;5)=2.3.5=30

Từ 2x=3y=5z=>2x/30=3y/30=5z/30=>x/15=y/10=z/6

theo t/c dãy tỉ số=nhau:

x/15=y/10=z/6=(x+y-z)/(15+10-6)=95/19=5

=>x/15=5=>x=75

y/10=5=>y=50

z/6=5=>z=30

 Vậy....

Việt: làm như you ko copy chắc

b) \(\frac{99}{98}-\frac{98}{97}+\frac{1}{97.98}=\frac{99}{98}-\frac{98}{97}+\frac{1}{97}-\frac{1}{98}=\left(\frac{99}{98}-\frac{1}{98}\right)-\left(\frac{98}{97}-\frac{1}{97}\right)\)

\(=\frac{98}{98}-\frac{97}{97}=1-1=0\)

Số bị chia:259

Số chia:125

\(2^n+3.2^n=144\)

\(=>2^n.\left(1+3\right)=144=>2^n.4=144=>2^n=144:4=36\)

Xem lại đề

3².3⁴.3ⁿ=3⁷

=>3²⁺⁴⁺ⁿ=3⁷

=>3⁶⁺ⁿ=3⁷

=>6+n=7=>n=1