Tính nhanh: 24. (-56) – 12. 88
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Đáp án:
12 + 24 + 88 + 76 + 10
= ( 12 + 88 ) + ( 24 + 76)
= 100 + 100
= 200
\(2\times169\times12-3\times68\times8-24\)
\(=24\times169-24\times68-24\)
\(=24\times\left(169-68-1\right)\)
\(=24\times100=2400\)
78 - 64 - 88 + 22 + 42 + 56
= 14 - 88 + 22 + 42 + 56
= -74 + 22 + 42 + 56
= -52 + 42 + 56
= -10 + 56
= 46
a, 56 x 12 + 24 x 36 + 24 x36 = 56 x 12 + 12 x 2 x 36 + 12 x 2 x 36
= 56 x 12 + 12 x 72 + 12 x 72
= 12 x ( 56 + 72 + 72 )
= 12 x 200
= 2400
b, 2005 x 178 - 77 x 2005 - 2005 = 2005 x ( 178 - 77 - 1 )
= 2005 x 100
= 200500
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1/2 x 2/3 x 3/4 x 4/5 x 5/6 x 6/7 x 7/8 x 8/9
1x2x3x4x5x6x7x8
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2x3x4x5x6x7x8x9
gạch các số giống nhau ở tử và mẫu và bằng 1/9
24 x 55 + 12 x 88 + 24
= ( 24 x 55 ) + ( 12 x 88 ) + 24
= 1320 + 1056 + 24
= 2376 + 24
= 2400
_HT_
\(24\cdot55+12\cdot88+24\)
\(\Leftrightarrow\left(24\cdot55\right)+\left(24\cdot88:2\right)+24\)
\(\Rightarrow\left(24+55\right)+\left(24\cdot44\right)+24\)
\(\Leftrightarrow24\left(55+44+1\right)\)
\(\Leftrightarrow24\cdot100\)
\(\Leftrightarrow2400\)
a) \(\dfrac{3}{8}+\dfrac{7}{12}+\dfrac{10}{16}+\dfrac{10}{24}\)
\(=\dfrac{3}{8}+\dfrac{7}{12}+\dfrac{5}{8}+\dfrac{5}{12}\)
\(=\left(\dfrac{3}{8}+\dfrac{5}{8}\right)+\left(\dfrac{7}{12}+\dfrac{5}{12}\right)\)
\(=1+1\)
\(=2\)
b) \(\dfrac{4}{6}+\dfrac{7}{13}+\dfrac{17}{9}+\dfrac{19}{13}+\dfrac{1}{9}+\dfrac{14}{6}\)
\(=\dfrac{2}{3}+\dfrac{7}{13}+\dfrac{17}{9}+\dfrac{19}{13}+\dfrac{1}{9}+\dfrac{7}{3}\)
\(=\left(\dfrac{2}{3}+\dfrac{7}{3}\right)+\left(\dfrac{7}{13}+\dfrac{19}{13}\right)+\left(\dfrac{17}{9}+\dfrac{1}{9}\right)\)
\(=3+2+2\)
\(=7\)
c) \(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}\)
\(=\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\)
\(=1-\dfrac{1}{7}\)
\(=\dfrac{6}{7}\)
=-2400