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a) = \(\frac{5\cdot7\cdot8\cdot9\cdot10}{7\cdot8\cdot9\cdot10\cdot11}\)
= \(\frac{5}{11}\) gạch hết các chữ số 7 , 8 , 9 , 10.
b) = \(\frac{3\cdot145+3\cdot55}{6\cdot215+6\cdot85}\)
= \(\frac{3\cdot\left(145+55\right)}{6\cdot\left(215+85\right)}\)
= \(\frac{3\cdot200}{6\cdot300}\)
= \(\frac{1}{3}\)
a) \(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}\)
\(=1-\frac{1}{4}\)
\(=\frac{3}{4}\)
b) \(\frac{4}{7}\times\frac{8}{9}+\frac{4}{7}\times\frac{1}{9}\)
\(=\frac{4}{7}\times\left(\frac{8}{9}+\frac{1}{9}\right)\)
\(=\frac{4}{7}\times1\)
\(=\frac{4}{7}\)
c) \(\frac{13}{6}\times\frac{3}{8}-\frac{3}{8}\times\frac{7}{6}\)
\(=\frac{3}{8}\times\left(\frac{13}{6}-\frac{7}{6}\right)\)
\(=\frac{3}{8}\times1\)
\(=\frac{3}{8}\)
d) \(\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+...+\frac{1}{9x10}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
\(=1-\frac{1}{10}\)
\(=\frac{9}{10}\)
( 45 - 5 x 9 ) x 1 x 2 x 3 x 4 x 5 x 6 x 7
= ( 45 - 45 ) x 1 x 2 x 3 x 4 x 5 x 6 x 7
= 0 x 1 x 2 x 3 x 4 x 5 x 6 x 7
= 0
( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 ) x ( 72 - 8 x 8 - 8 )
= ( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 ) x ( 72 - 64 - 8 )
= ( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 ) x 0
= 0
2 x 5 + 5 x 7 + 9 x 3
= 5 x ( 2 + 7 ) + 9 x 3
= 5 x 9 + 9 x 3
= 9 x ( 5 + 3 )
= 9 x 8
= 72
2x5+5x9+9x3 15:5+27:5+8:5
=9x5+(2+3) =(15+8+27):5
=9x5+5 =50:5
=9x5+5x1 =10
=5x(9+1) 9+9x3+18:2x6
=5x10 =1x9+9x3+9x6
=50 =9x(1+6+3)
=9x10
=90
1. 36 x 25 x 4
= 36 x 100
= 3600
2. 18 x 24 : 9
= 18 : 9 x 24
= 2 x 24
= 48
3. 41 x 2 x 8 x 5
= 41 x 8 x 10
= 328 x 10
= 3280
4. 108 x (23+7)
= 108 x 30
= 3240
5. 215 x 86 + 215 x 14
= 215 x (86+14)
= 215 x 100
= 21500
6. 53 x 128 - 43 x 128
= (53-43) x 128
= 10 x 128
= 1280