1/40x1/30x1/20x1/12x1/6x1/2
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\(1\frac{1}{3}.1\frac{1}{4}.1\frac{1}{5}.1\frac{1}{6}.1\frac{1}{7}.1\frac{1}{8}=\frac{4}{3}.\frac{5}{4}.\frac{6}{5}.\frac{7}{6}.\frac{8}{7}.\frac{9}{8}=\frac{9}{3}=3\)
Mik có sửa lại đề
C1: \(\dfrac{6}{7}\times\dfrac{1}{2}+\dfrac{6}{7}\times\dfrac{1}{4}=\dfrac{3}{7}+\dfrac{3}{14}=\dfrac{6}{14}+\dfrac{3}{14}=\dfrac{9}{14}\)
C2:\(\dfrac{6}{7}\times\dfrac{1}{2}+\dfrac{6}{7}\times\dfrac{1}{4}=\dfrac{6}{7}\times\left(\dfrac{1}{2}+\dfrac{1}{4}\right)=\dfrac{6}{7}\times\left(\dfrac{2}{4}+\dfrac{1}{4}\right)=\dfrac{6}{7}\times\dfrac{3}{4}=\dfrac{9}{14}\)
4/6x1/5+4/6x3/5+2/6x4/5
=4/6.(1/5+3/5)+2/6.4/5
=4/6.4/5+2/6.4/5
=4/5.(4/6+2/6)
=4/5.6/6
=4/5.1 = 4/5
\(\frac{1}{20}\cdot\frac{1}{30}\cdot\frac{1}{42}\cdot\frac{1}{56}\cdot\frac{1}{72}\cdot\frac{1}{90}\)
\(=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{4}-\frac{1}{10}\)
\(=\frac{6}{40}=\frac{3}{20}\)
ta có
a=1+2+4+...........+211
=1+[1+1]+[1+1+2]+[1+1+2+3]+..................+[1+1+2+3+...........+20] =[1+......+1]+[1+1+.....+1]+[2+......+2]+..........+[19+19]+20
=21+[1*20+2*19+...............+19*2+20]
=1561
\(\frac{1}{40}\)x\(\frac{1}{30}\)x\(\frac{1}{20}\)x\(\frac{1}{12}\)x\(\frac{1}{6}\)x\(\frac{1}{2}\)
= \(\frac{1}{40.30.20.12.6.2}\)
= \(\frac{1}{3456000}\)
k mik nha! (kb nhé!!!)