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\(a,\frac{2}{3}.\frac{4}{5}+\frac{1}{3}.\frac{2}{5}\)
= \(\frac{2}{5}.\frac{4}{3}+\frac{1}{3}.\frac{2}{5}=\frac{2}{5}\left(\frac{4}{3}+\frac{1}{3}\right)=\frac{2}{5}.\frac{5}{3}=\frac{2}{3}\)
\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}=\frac{1}{2}+\left(-\frac{1}{3}+\frac{1}{3}\right)+\left(-\frac{1}{4}+\frac{1}{4}\right)-\frac{1}{5}=\frac{1}{2}-\frac{1}{5}=\frac{3}{10}\)
= 1/2 - 1/5 = 2/5 (giải thích chút nha -1/3+ 1/3 = 0 ; -1/4 + 1/4 = 0 và sau khi quy đồng ta được kq như tớ đã tính)
1)
a) 21/23 x 5/11 x 4/35
= \(\frac{21.5.4}{23.11.35}\)
= \(\frac{7.3.5.4}{23.11.5.7}\)
= \(\frac{3.4}{23.11}\) ( ta đã rút gọc 7 và 5 )
= \(\frac{12}{253}\)
b) 26 x 13/42 : 13
= \(\frac{26.13}{42.13}\)
= \(\frac{13.2.13}{21.2.13}\)
= \(\frac{13}{21}\)
c) 5/9 : 2/9 + 4/9 : 2/3
= \(\frac{5.9}{9.2}+\frac{4.3}{9.2}\)
= \(\frac{5}{2}+\frac{2.2.3}{3.3.2}\)
= \(\frac{5}{2}+\frac{2}{3}\)
= \(\frac{19}{6}\)
2 ) tìm x
a ) 2 - x = 1/3 + 1/6
2 - x = 1/2
x = 2 - 1/2
x = 3/2
b ) x + 5/8 = 1 + 3/4
x + 5/8 = 7/4
x = 7/4 - 5/8
x = 9/8
3)
a) x < 3/4 + 4/5
=> x < 31/20
=> x = 1
b) 1/2 + 3/5 < x < 5/4
=> 11/10 < x < 5/4
ta quy đồng phân số 11/10 và 5/4
\(\frac{11}{10}=\frac{11.4}{10.4}=\frac{44}{40}\)
\(\frac{5}{4}=\frac{5.10}{4.10}=\frac{50}{40}\)
=> 44/40 < x < 50/40
=> x không thỏa mãn
\(\frac{5}{4}+\frac{7}{12}+\frac{7}{5}+\frac{75}{100}+\frac{40}{25}+\frac{20}{48}\)
\(=\frac{5}{4}+\frac{7}{12}+\frac{7}{5}+\frac{3}{4}+\frac{8}{5}+\frac{5}{12}\)
\(=\left(\frac{5}{4}+\frac{3}{4}\right)+\left(\frac{7}{12}+\frac{5}{12}\right)+\left(\frac{7}{5}+\frac{8}{5}\right)\)
\(=\frac{8}{4}+\frac{12}{12}+\frac{15}{5}\)
\(=2+1+3\)
\(=6\)
Đặt A=5/4 + 7/12 + 7/5 + 75/100 + 40/25 + 20/48
=>A=125/100 + 28/48 + 35/25 + 75/100 + 40/25 + 20/48
=>A=(125/100 + 75/100) + (28/48 + 20/48) + (35/25 + 40/25)
=>A= 2 + 1 + 3
=>A= 6
Vậy A=6
ta có 1/2 - 1/3 + 1/4 -1/6 = 1/6 +1/4 - 1/6 = 1/4 ( giải thích chút nha : hiệu của 1/2 với 1/3 bằng 1/6)
a) \(\frac{2}{3}+\frac{3}{4}+\frac{4}{5}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}\)
= \(\left(\frac{2}{3}+\frac{1}{3}\right)+\left(\frac{3}{4}+\frac{1}{4}\right)+\left(\frac{4}{5}+\frac{1}{5}\right)\)
= \(\frac{3}{3}+\frac{4}{4}+\frac{5}{5}\)
= \(1+1+1\)
= \(3\)
b) \(\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{4}{6}+\frac{9}{12}+\frac{16}{20}\)
= \(\left(\frac{1}{3}+\frac{4}{6}\right)+\left(\frac{1}{4}+\frac{9}{12}\right)+\left(\frac{1}{5}+\frac{16}{20}\right)\)
= \(\left(\frac{1}{3}+\frac{2}{3}\right)+\left(\frac{1}{4}+\frac{3}{4}\right)+\left(\frac{1}{5}+\frac{4}{5}\right)\)
= \(\frac{3}{3}+\frac{4}{4}+\frac{5}{5}\)
= \(1+1+1\)
= \(3\)
ko sai được... đúng 100%
a)2/3+3/4+4/5+1/3+1/4+1/5
= ( 2/3 + 1/3 ) + ( 3/4 + 1/4 ) + ( 4/5 + 1/5 )
= 1 + 1 + 1 = 3
b) 1/3 + 1/4 + 1/5 + 4/6 + 9/12 + 16/20
= 1/3 + 1/4 + 1/5 + 2/3 + 3/4 4/5
= ( 1/3 + 2/3 ) + ( 1/4 + 3/4 ) + ( 1/5 + 4/5 )
= 1 + 1 + 1 = 3