Tính thuận tiện:
5.2\(^3\) + 7\(^9\) : 7\(^7\) - 1\(^{2020}\)
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\(=\dfrac{2}{5}\cdot\dfrac{7}{3}\cdot\dfrac{3}{7}\cdot\dfrac{5}{2}+2020=1+2020=2021\)
Bài 1:
1+2+3+4+5+6+7+8+9 = (1+9) + (2+8) + (3+7) + (4+6) + 5
= 10 + 10 + 10 + 10 + 5
= 45
Bài 2:
7+7+7+7+7+7+7+7+7+7+7 = 7x11
= 77
Bài 3:
Bố = mẹ thì mẹ = bố
nhớ kích nha
(1+9)+(2+8)+(3+7)+(4+6)+5
=10+10+10+10+5
đếm xem có bao nhiêu số 7 rồi nhân với 7
\(\dfrac{2}{3}+\dfrac{7}{13}-\dfrac{17}{9}+\dfrac{19}{13}-\dfrac{1}{9}+\dfrac{7}{3}\)
\(=\left(\dfrac{2}{3}+\dfrac{7}{3}\right)+\left(\dfrac{7}{13}+\dfrac{19}{13}\right)-\left(\dfrac{17}{9}+\dfrac{1}{9}\right)\)
\(=\dfrac{9}{3}+\dfrac{26}{13}-\dfrac{18}{9}\)
\(=3+2-2\)
\(=3\)
\(\frac{5}{6}.\frac{8}{7}+\frac{5}{7}.\frac{2}{3}\)
\(=\frac{5}{7}.\frac{8}{6}+\frac{5}{7}.\frac{2}{3}\)
\(=\frac{5}{7}\left(\frac{8}{6}+\frac{2}{3}\right)\)
\(=\frac{5}{7}\left(\frac{8}{6}+\frac{4}{6}\right)\)
\(=\frac{5}{7}.2=\frac{10}{7}\)
\(=\dfrac{3}{7}\left(\dfrac{5}{4}-\dfrac{2}{9}\right)=\dfrac{3}{7}\cdot\dfrac{45-8}{36}=\dfrac{3}{36}\cdot\dfrac{37}{7}=\dfrac{37}{84}\)
\(\dfrac{3}{7}:\dfrac{4}{5}-\dfrac{3}{7}:\dfrac{9}{2}\)
\(=\dfrac{3}{7}\times\dfrac{5}{4}-\dfrac{3}{7}\times\dfrac{2}{9}\)
\(=\dfrac{3}{7}\times\left(\dfrac{5}{4}-\dfrac{2}{9}\right)\)
\(=\dfrac{3}{7}\times\left(\dfrac{45}{36}-\dfrac{8}{36}\right)\)
\(=\dfrac{3}{7}\times\dfrac{37}{36}\)
\(=\dfrac{37}{84}\)
#DatNe
a)= 2021.2021-2020.(2021+1)
= 2021.(2020+1)-2020.(2021+1)
= (2021.2020)+2021-(2020.2021)-2020
= 1
b) B= (1+2-3-4)+(5+6-7-8)+(9+10-11-12)...........+(2017+2018-2019-2020)+2021
B= -4+(-4)+....................(-4)+2021
B= -4x505+2021
B= -2020 + 2021
B = 1
\(\left(\dfrac{2}{7}+\dfrac{5}{7}\right)+\left(\dfrac{3}{5}+\dfrac{2}{5}\right)+\left(\dfrac{3}{8}+\dfrac{4}{8}+\dfrac{1}{8}\right)+\left(\dfrac{1}{9}+\dfrac{8}{9}\right)=1+1+1+1=4\)
\(5\cdot2^3+7^9:7^7-1^{2020}\\ =5\cdot8+7^2-1\\ =40+49-1\\ =89-1\\ =88\)
\(5.2^3+7^9:7^7-1^{2020}\)
\(=5.8+7^2-1\)
\(=40+49-1\)
\(=88\)