Bài 5
a) $\frac{3n+2}{n-1}$ = $\frac{3(n-1)+5}{n-1}$ = 3+ $\frac{5}{n-1}$
Để A nguyên thì 5 chia hết cho n-1
⇒ n-1 ∈ Ư(5) = {-1 ; 1 ; -5 ; 5 }
bạn tự lập bảng nhé
b) $\frac{3²}{3.14}$ + $\frac{3²}{7.18}$ + $\frac{3²}{9.22}$ + $\frac{3²}{11.26}$ + $\frac{3²}{13.30}$
= 3²($\frac{1}{3.14}$ + $\frac{1}{7.18}$ + $\frac{1}{9.22}$ + $\frac{1}{11.26}$ + $\frac{1}{13.30}$)
= 9 . $\frac{1}{2}$ ($\frac{1}{3.7}$ + $\frac{1}{7.9}$ + $\frac{1}{9.11}$ + $\frac{1}{11.13}$ + $\frac{1}{13.15}$)
= $\frac{9}{2}$ . $\frac{1}{2}$ ($\frac{2}{3.7}$ + $\frac{2}{7.9}$ + $\frac{2}{9.11}$ + $\frac{2}{11.13}$ + $\frac{2}{13.15}$)
= $\frac{9}{4}$ ($\frac{1}{3}$ - $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{11}$ + $\frac{1}{11}$ - $\frac{1}{13}$ + $\frac{1}{13}$ - $\frac{1}{15}$)
= $\frac{9}{4}$ ( $\frac{1}{3}$ - $\frac{1}{15}$ )
= $\frac{9}{4}$ . $\frac{4}{15}$
= $\frac{3}{5}$
