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1 . tính nhẩm
2,4 x 10 = 24
1,54 x 10 = 15,4
3,8 x100 = 280
1,05 x 100 = 105.
4,86 x 1000 = 4860
0,458 x 1000 = 458
1.
2,4 x 10 = 24
1,54 x 10 = 15, 4
3,8 x 100 = 380
1,05 x 100 =105
4,86 x 1000 = 4860
0, 458 x 1000 = 458
C=1x2+2x3+3x4 +......+99x100
<=>3xC=1x2x3+2x3x3+.....+99x100x3
<=>3xC=1x2x3+2x3x(4-1)+.....+99x100x(101-98)
<=>3xC=1x2x3-1x2x3+2x3x4-2x3x4+......-98x99x100+99x100x101
<=>3xC=99x100x101 <=> C=99x100x101 :3 <=> C= 333300
\(C=1.2+2.3+3.4+...+99.100\)
=> 3C = 1.2.3 + 2.3.3 + 3.4.3 + ...+ 99.100.3
3C = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + ...+ 99.100.(101-98)
3C = 1.2.3 + 2.3.4 -1.2.3 + 3.4.5 - 2.3.4 + ...+ 99.100.101-98.99.100
\(3C=\left(1.2.3+2.3.4+3.4.5+...+99.100.101\right)-\left(1.2.3+2.3.4+...+98.99.100\right)\)
\(3C=99.100.101\Rightarrow C=\frac{99.100.101}{3}=333300\)
Dấu . là dấu nhân
3C = 3 x C
a) 25 : 0,1= 2,5 48 : 0,01= 0,48 95: 0,1=9,5
25 x 10=250 48 x 100=4800 72:0,01= 0,72
a) 25 : 0,1 = 250.
25 x 10 = 250
48 : 0,01 = 4800
48 x 100 = 4800
95 : 0,1 = 950
72 : 0,01 = 7200.
b) 11 : 0,25 = 44
11 x 4 = 44
32 : 0,5 = 64
32 x 2 = 64
75 : 0,5 =150
125 : 0,25 = 500.
\(\frac{19,01\div0,1\times\left(208\times9+208\right)}{2,08\times100\div0,01}\)
\(=\frac{190,1\times\left(208\times9+208\times1\right)}{208\div0,01}\)
\(=\frac{190,1\times\left[208\times\left(9+1\right)\right]}{20800}\)
\(=\frac{190,1\times\left[208\times10\right]}{20800}\)
\(=\frac{190,1\times2080}{20800}\)
\(=\frac{190,1\times10\times208}{100\times208}\)
\(=\frac{1901}{100}\)
\(\frac{19,01\div0,1\times(208\times9+208)}{2,08\times100\div0,01}\)
\(=\frac{19,01\div\frac{1}{10}\times(208\times9+208\times1)}{208\div\frac{1}{100}}\)
\(=\frac{19,01\times10\times\left[208\times(9+1)\right]}{208\times100}\)
\(=\frac{19,01\times100\times208\times10}{208\times100}=19,01\times10=190,1\)
Lời giải :
\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{99\cdot100}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{99}-\frac{1}{100}\)
\(=1-\frac{1}{100}\)
\(=\frac{99}{100}\)
ko chép lại đề :
= \(\frac{1}{1}\)- \(\frac{1}{2}\)+ \(\frac{1}{2}\)- \(\frac{1}{3}\)+ \(\frac{1}{3}\)- \(\frac{1}{4}\)+ ......... + \(\frac{1}{98}\)- \(\frac{1}{99}\)+ \(\frac{1}{99}\)- \(\frac{1}{100}\)
= \(1-\frac{1}{100}\)
= \(\frac{99}{100}\)
a)3,25 x 10 = 32,5 b)417,56 x 100 = 41756 c)28,5 x 100 = 285 3,25 x 0,1 = 0,325 417,56 x 0,01 = 4,1756 28,5 x 0,01 = 0,285