a. Có 5+4+3+2+1= 15 đoạn

Nam có số viên bi là:

(20 + 31 - 5) : 2 -5 = 18 ( viên bi)

Đáp số : 18 viên bi

18 viên bi

\(\dfrac{18}{36}=\dfrac{18:18}{36:18}=\dfrac{1}{2}=\dfrac{1.5}{2.5}=\dfrac{5}{10}\)

\(\dfrac{14}{35}=\dfrac{14:7}{35:7}=\dfrac{2}{5}=\dfrac{2.2}{5.2}=\dfrac{4}{10}\)

\(\dfrac{27}{45}=\dfrac{27:9}{45:9}=\dfrac{3}{5}=\dfrac{3.2}{5.2}=\dfrac{6}{10}\)

\(\dfrac{40}{50}=\dfrac{40:10}{50:10}=\dfrac{4}{5}=\dfrac{4.2}{5.2}=\dfrac{8}{10}\)

Lưu ý : \(.\) = \(\times\)

hơi dài 

\(A=\left(\dfrac{1}{2}-1\right)\left(\dfrac{1}{3}-1\right)...\left(\dfrac{1}{10}-1\right)=\left(\dfrac{1-2}{2}\right)\left(\dfrac{1-3}{3}\right)...\left(\dfrac{1-10}{10}\right)=\dfrac{-1}{2}.\dfrac{-2}{3}...\dfrac{-9}{10}\)

Vì có tất cả 9 (lẻ) thừa số hạng nên A sẽ có dấu âm
\(A=\dfrac{-1}{2}.\dfrac{-2}{3}...\dfrac{-9}{10}=-\left(\dfrac{1}{2}.\dfrac{2}{3}...\dfrac{9}{10}\right)=\dfrac{-1}{10}\)

Vậy \(A=\dfrac{-1}{10}\)

bấm máy tính đê

tự tính

\(5^{10}+5^{11}+5^{12}\)

\(=5\left(5^9+5^{10}+5^{11}\right)⋮5\)

⇒ \(5^{10}+5^{11}+5^{12}⋮5\)

5¹⁰ chia hết cho 5

5¹¹ chia hết cho 5

5¹² chia hết cho 5

=> 5¹⁰ + 5¹¹ + 5¹² chia hết cho 5

nhanh lên nha mik gấp lắm

\(a.\)\(\dfrac{5}{10}=\dfrac{5:5}{10:5}=\dfrac{1}{2};\dfrac{25}{75}=\dfrac{25:25}{75:25}=\dfrac{1}{3}.\)

Quy đồng: \(\dfrac{1}{2}=\dfrac{1\times3}{2\times3}=\dfrac{3}{6};\dfrac{1}{3}=\dfrac{1\times2}{3\times2}=\dfrac{2}{6}.\)

\(b.\)\(\dfrac{42}{56}=\dfrac{42:14}{56:14}=\dfrac{3}{4};\dfrac{18}{48}=\dfrac{18:6}{48:6}=\dfrac{3}{8}.\)

Quy đồng: \(\dfrac{3}{4}=\dfrac{3\times2}{4\times2}=\dfrac{6}{8};\) Giữ nguyên \(\dfrac{3}{8}.\)