Tính bằng cách thuận tiện nhất
13,8:0,5+13,8:0,25+13,8:0,125
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\(\dfrac{21}{17}\times\dfrac{13}{14}\times56\times\dfrac{3}{42}\)
\(=\dfrac{21}{17}\times\dfrac{13}{14}\times56\times\dfrac{1}{14}\)
\(=\dfrac{21\times13\times56}{17\times14\times14}\)
\(=\dfrac{3\times7\times13\times14\times2\times2}{17\times2\times7\times14}\)
\(=\dfrac{3\times13\times2}{17}\)
\(=\dfrac{78}{17}\)
#Sahara |
A = 1/1 - 1/2 + 1/3 - 1/3 + 1/4
A = 1/1 - 1/4
A = 3/4
vậy A = 3/4
\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}\)
\(=1-\dfrac{1}{4}=\dfrac{4-1}{4}=\dfrac{3}{4}\)
\(101\times1,996-1,996\)
\(=\left(101-1\right)\times1,996\)
\(=100\times1,996\)
\(=199,6\)
#Sahara |
101 x 1996 - 1996
= 101 x 1996 - 1996 x 1
= 1996 x (101 - 1)
= 1996 x 100
= 199600
\(\dfrac{1}{42}+\dfrac{1}{30}+\dfrac{1}{12}+\dfrac{1}{6}+\dfrac{1}{2}\)
\(=\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{30}+\dfrac{1}{42}\)
\(=\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+\dfrac{1}{3\times4}+\dfrac{1}{5\times6}+\dfrac{1}{6\times7}\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\)
\(=1-\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{7}\)
\(=\dfrac{140}{140}-\dfrac{35}{140}+\dfrac{28}{140}-\dfrac{20}{140}\)
\(=\dfrac{113}{140}\)
#Sahara |
\(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{30}+\dfrac{1}{42}\)
\(=\dfrac{1}{2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+\dfrac{1}{6.7}\)
\(=\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\)
\(=1-\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{7}=\dfrac{140-35+28-20}{140}=\dfrac{113}{140}\)
Mình nghĩ đề bài cần thêm 1/20 mới tính nhanh được
\(\dfrac{5}{3}< x< \dfrac{7}{3}\)
\(\dfrac{5}{3}< \dfrac{x}{3}< \dfrac{7}{3}\)
\(x=\dfrac{6}{3}\)
\(x=2\)
\(13,8:0,5+13,8:0,25+13,8:0,125\)
\(=13,8\times2+13,8\times4+13,8\times8\)
\(=13,8\times\left(2+4+8\right)\)
\(=13,8\times14\)
\(=193,2\)
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