B nha
Giải:
a) Để \(A=\dfrac{3n+2}{n-1}\) là số nguyên thì \(3n+2⋮n-1\)
\(3n+2⋮n-1\)
\(\Rightarrow3n-3+5⋮n-1\)
\(\Rightarrow5⋮n-1\)
\(\Rightarrow n-1\inƯ\left(5\right)=\left\{\pm1;\pm5\right\}\)
Ta có bảng giá trị:
| n-1 | -5 | -1 | 1 | 5 |
| n | -4 | 0 | 2 | 6 |
Vậy \(n\in\left\{-4;0;2;6\right\}\)
b) \(M=\dfrac{3^2}{3.14}+\dfrac{3^2}{7.18}+\dfrac{3^2}{9.22}+\dfrac{3^2}{11.26}+\dfrac{3^2}{13.30}\)
\(M=\dfrac{9}{2}.\left(\dfrac{1}{3.7}+\dfrac{1}{7.9}+\dfrac{1}{9.11}+\dfrac{1}{11.13}+\dfrac{1}{13.15}\right)\)
\(M=\dfrac{9}{2}.\dfrac{1}{2}.\left(\dfrac{2}{21}+\dfrac{2}{7.9}+\dfrac{2}{9.11}+\dfrac{2}{11.13}+\dfrac{2}{13.15}\right)\)
\(M=\dfrac{9}{4}.\left(\dfrac{2}{21}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}\right)\)\(M=\dfrac{9}{4}.\left(\dfrac{2}{21}+\dfrac{1}{7}-\dfrac{1}{15}\right)\)
\(M=\dfrac{9}{4}.\dfrac{6}{35}\)
\(M=\dfrac{27}{70}\)
D nha