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Lời giải:
$1+2+3+....+25$
$=(1+25)+(2+24)+(3+23)+(4+22)+(5+21)+(6+20)+(7+19)+(8+18)+(9+17)+(10+16)+(11+15)+(12+14)+13$
$=\underbrace{26+26+26+...+26}_{12}+13$
$=26\times 12+13=325$
![](https://rs.olm.vn/images/avt/0.png?1311)
= (26/15-7/15):19/45
=\(\dfrac{19}{15}:\dfrac{19}{45}\)
=\(\dfrac{19}{15}x\dfrac{45}{19}=\dfrac{45}{15}=3\)
![](https://rs.olm.vn/images/avt/0.png?1311)
3 + 6 + 9 + 12 + 15
= ( 3 +12) + ( 6 + 9) +15
= 15 + 15 + 15
= 15 x 3
= 45
![](https://rs.olm.vn/images/avt/0.png?1311)
a,Ta có: \(1-\frac{17}{19}=\frac{2}{19}\) và \(1-\frac{15}{17}=\frac{2}{17}\)
Vì \(\frac{2}{19}< \frac{2}{17}\) nên \(\frac{17}{19}< \frac{15}{17}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) \(\frac{7}{18}+\frac{4}{23}+\frac{11}{18}+\frac{19}{23}\)
\(=\left(\frac{7}{18}+\frac{11}{18}\right)+\left(\frac{4}{23}+\frac{19}{23}\right)\)
\(=1+1\)
\(=2\)
b) \(\frac{19}{15}+\frac{31}{23}-\frac{4}{15}-\frac{8}{23}\)
\(=\left(\frac{19}{15}-\frac{4}{15}\right)+\left(\frac{31}{23}-\frac{8}{23}\right)\)
\(=1+1\)
\(=2\)
ủng hộ tớ nha
![](https://rs.olm.vn/images/avt/0.png?1311)
tính = cách thuận tiện nhất
a/ 5/11 nhân 7/25 trừ 15/11 nhân 1/5
b/ 3/7 nhân 25/19 trừ 1/7 nhân 18/19
![](https://rs.olm.vn/images/avt/0.png?1311)
a ) \(\frac{5}{11}\times\frac{7}{25}\times\frac{15}{11}\times\frac{1}{5}\)
\(=\frac{5\times7\times15\times1}{11\times25\times11\times5}\)
\(=\frac{1\times7\times3\times1}{11\times5\times11\times1}\)
\(=\frac{21}{\left(10\times5+5\times1\right)\times11}\)
\(=\frac{21}{55\times11}\)
\(=\frac{21}{55\times10+55}\)
\(=\frac{21}{550+55}\)
\(=\frac{21}{605}\) ( quá đơn giản )
\(\)
A) \(\frac{5}{11}\times\frac{7}{25}-\frac{15}{11}\times\frac{1}{5}\)
\(=\frac{5\times7}{11\times25}-\frac{15\times1}{11\times5}\)
\(=\frac{35}{10\times25+25}-\frac{15}{55}\)
\(=\frac{35}{250+25}-\frac{15}{55}\)
\(=\frac{35}{275}-\frac{15}{55}\)
\(=\frac{7}{55}-\frac{15}{55}\)
\(=\frac{7-15}{55}\)
\(=\frac{-8}{55}\)
= (26/15-7/15):19/45
=\(\dfrac{19}{15}:\dfrac{19}{45}\)
=\(\dfrac{19}{15}x\dfrac{45}{19}=\dfrac{45}{15}=3\)
\(\dfrac{26}{15}\) : \(\dfrac{19}{45}\) - \(\dfrac{7}{15}\) : \(\dfrac{19}{45}\)=( \(\dfrac{26}{15}\) - \(\dfrac{7}{15}\)) : \(\dfrac{19}{45}\)
= \(\dfrac{19}{15}\) : \(\dfrac{19}{45}\)
= 3