Tính nhanh: 5/8+5/24+5/48+...+5/200+5
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48 + 49 - 9 - 8 = (48 -8) + (49 – 9) = 40 + 40 = 80
24 + 65 - 4 – 5 = (24 – 4) + (65 – 5) = 20 + 60 = 80
Học sinh nhẩm và ghi:
8 x 5 = 40
8 x 4 = 32
8 x 6 = 48
8 x 3 = 24
40 : 8 = 5
32 : 8 = 4
48 : 8 = 6
24 : 8 = 3
40 : 5 = 8
32 : 4 = 8
48 : 6 = 8
24 : 3 = 8
(chú ý: mối quan hệ giữa nhân và chia)
Tính nhẩm:
8 x 5 = 40
8 x 4 = 32
8 x 6 = 48
8 x 3 = 24
40 : 8 = 5
32 : 8 = 4
48 : 8 = 6
24 : 8 = 3
40 : 5 = 8
32 : 4 = 8
48 : 6 = 8
24 : 3 = 8
2. Tính (+5) + ( +4) = 9
3.Tính (- 9) + ( - 1) = -10
4.Tính : 28 + (-15) = 13
5.Tính (-15) + 12 = - 3
6.Tính (-25) + 25 = 0
7.Tính : 13 - 17 = - 4
8.Tính : (-78) - 9 = - 87
9.Tính : ( -24) - ( - 16) = - 8
10.Tính : 17 - (-3) = 20
11.Tính nhanh : (-21) - ( 48 + 52 - 21) = -100
2, (+5)+(+4)=+9 hoạc 9 cũng được
3, (-9)+(-1)=-10
4, 28+(-15)=13
5, (-15)+12=-3
6, (-25)+25=0
7, 13-17=4
8, (-78)-9=-87
9, (-24)-(-16)=-40
10, 17-(-3)=20
11, (-21+21)-(48-52)=-4
2/3 + 7/1 = 2/3 + 7 = 2/3 + 21/3 = 23/3
25/48 + 11/24 = 25/48 + 22/48 = 47/48
5/7 + 3/8 = 40/56 + 21/56 = 61/56
15/24 + 12/6 = 5/8 + 2 = 5/8 + 16/8 = 21/8
5/6 + 4/3 = 5/6 + 8/6 = 13/6
3/8 + 7/12 = 9/24 + 14/24 = 23/24
\(\dfrac{5}{8}+\dfrac{5}{24}+\dfrac{5}{48}+...+\dfrac{5}{528}\)
\(=\dfrac{5}{2.4}+\dfrac{5}{4.6}+\dfrac{5}{6.8}+...+\dfrac{5}{22.24}\)
\(=\dfrac{5}{2}.\left(\dfrac{2}{2.4}+\dfrac{2}{4.6}+\dfrac{2}{6.8}+...+\dfrac{2}{22.24}\right)\)
\(=\dfrac{5}{2}.\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-...-\dfrac{1}{22}+\dfrac{1}{22}-\dfrac{1}{24}\right)\)
\(=\dfrac{5}{2}.\left(\dfrac{1}{2}-\dfrac{1}{24}\right)\)
\(=\dfrac{5}{2}.\dfrac{11}{24}\)
\(=\dfrac{55}{48}\).
58+524+548+...+552858+524+548+...+5528
A=52.4+54.6+56.8+...+522.24=52.4+54.6+56.8+...+522.24
=52.(22.4+24.6+26.8+...+222.24)=52.(22.4+24.6+26.8+...+222.24)
=52.(12−14+14−16+16−...−122+122−124)=52.(12−14+14−16+16−...−122+122−124)
=52.(12−124)=52.(12−124)
=52.1124=52.1124
=5548=5548.
vậy A = 5548
Ta có: \(A=\dfrac{5}{8}+\dfrac{5}{24}+\dfrac{5}{48}+...+\dfrac{5}{9800}\)
\(=\dfrac{5}{2}\left(\dfrac{2}{8}+\dfrac{2}{24}+\dfrac{2}{48}+...+\dfrac{2}{9800}\right)\)
\(=\dfrac{5}{2}\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{98}-\dfrac{1}{100}\right)\)
\(=\dfrac{5}{2}\left(\dfrac{1}{2}-\dfrac{1}{100}\right)\)
\(=\dfrac{5}{2}\cdot\dfrac{49}{50}\)
\(=\dfrac{245}{100}=\dfrac{49}{20}\)
32 : 8 = 4
24 : 8 = 3
40 : 5 = 8
16 : 8 = 2
42 : 7 = 6
36 : 6 = 6
48 : 8 = 6
48 : 6 = 8
A = \(\dfrac{5}{8}+\dfrac{5}{24}+\dfrac{5}{48}+....+\dfrac{5}{200}\) + 5
A = \(\dfrac{5}{2}\) x ( \(\dfrac{2}{8}\) +\(\dfrac{2}{24}\) +\(\dfrac{2}{48}\) + ......+\(\dfrac{2}{100}\)) + 5
A = \(\dfrac{5}{2}\)x ( \(\dfrac{2}{2\times4}\) + \(\dfrac{2}{4\times6}\) +.....+\(\dfrac{2}{10\times20}\)) + 5
A = \(\dfrac{5}{2}\) x ( \(\dfrac{1}{2}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{4}\) - \(\dfrac{1}{6}\) +........+ \(\dfrac{1}{10}\) - \(\dfrac{1}{20}\) ) + 5
A = \(\dfrac{5}{2}\) x ( \(\dfrac{1}{2}\)- \(\dfrac{1}{20}\)) + 5
A = \(\dfrac{5}{2}\)x \(\dfrac{9}{20}\) + 5
A = 49/8