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3x . 2 + 15 = 33
3x . 2 = 33 - 15 = 18
3x = 18 : 2 = 9 = 32
=> x = 2
a: 0,4m=4dm
Tỉ số là 16/4=4
b: 3/5m=60cm
Tỉ số là 60/480=1/8
c: 4/5kg=800g
Tỉ số là 100/800=1/8
d: 7,5hm=750m
Tỉ số là 60/750=2/25
e: 4/3m=80cm
Tỉ số là 80/60=4/3
f: 0,3 tạ=30kg
Tỉ số là 10/30=1/3
Bài 2:
n) Ta có: \(N=\dfrac{4}{2\cdot4}+\dfrac{4}{4\cdot6}+\dfrac{4}{6\cdot8}+...+\dfrac{4}{2014\cdot2016}\)
\(=2\left(\dfrac{2}{2\cdot4}+\dfrac{2}{4\cdot6}+\dfrac{2}{6\cdot8}+...+\dfrac{2}{2014\cdot2016}\right)\)
\(=2\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{2014}-\dfrac{1}{2016}\right)\)
\(=2\cdot\left(\dfrac{1}{2}-\dfrac{1}{2016}\right)\)
\(=2\cdot\dfrac{1007}{2016}=\dfrac{1007}{1008}\)
o) Ta có: \(\dfrac{1}{18}+\dfrac{1}{54}+\dfrac{1}{108}+...+\dfrac{1}{990}\)
\(=\dfrac{1}{3\cdot6}+\dfrac{1}{6\cdot9}+\dfrac{1}{9\cdot12}+...+\dfrac{1}{30\cdot33}\)
\(=\dfrac{1}{3}\left(\dfrac{3}{3\cdot6}+\dfrac{3}{6\cdot9}+\dfrac{3}{9\cdot12}+...+\dfrac{3}{30\cdot33}\right)\)
\(=\dfrac{1}{3}\left(\dfrac{1}{3}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{12}+...+\dfrac{1}{30}-\dfrac{1}{33}\right)\)
\(=\dfrac{1}{3}\left(\dfrac{1}{3}-\dfrac{1}{33}\right)\)
\(=\dfrac{1}{3}\cdot\dfrac{10}{33}=\dfrac{10}{99}\)
a) Ta có: \(\left(6\dfrac{4}{9}+3\dfrac{7}{11}\right)-4\dfrac{4}{9}\)
\(=\dfrac{58}{9}+\dfrac{40}{11}-\dfrac{40}{9}\)
\(=2+\dfrac{40}{11}=\dfrac{62}{11}\)
Bài 2:
b) Ta có: \(10\dfrac{1}{5}-5\dfrac{1}{2}\cdot\dfrac{60}{11}+3:15\%\)
\(=\dfrac{51}{5}-\dfrac{11}{2}\cdot\dfrac{60}{11}+3:\dfrac{3}{20}\)
\(=\dfrac{51}{5}-30+20\)
\(=\dfrac{51}{5}-10=\dfrac{1}{5}\)
c) Ta có: \(\dfrac{2}{3\cdot5}+\dfrac{2}{5\cdot7}+\dfrac{2}{7\cdot9}+...+\dfrac{2}{97\cdot99}\)
\(=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{97}-\dfrac{1}{99}\)
\(=\dfrac{1}{3}-\dfrac{1}{99}\)
\(=\dfrac{32}{99}\)
\(2+x:5=-3\\\Rightarrow x:5=-3-2\\\Rightarrow x:5=-5\\\Rightarrow x=-5\cdot5=-25\)
\(2+x:5=-3\)
\(=>x:5=\left(-3\right)-2\)
\(=>x:5=-5\)
\(=>x=\left(-5\right):5\)
\(=>x=-1\)
4: \(=29\cdot19-29\cdot13-19\cdot29+19\cdot13=-10\cdot13=-130\)
5: \(=31\left(-18-81-1\right)=-31\cdot100=-3100\)
6: \(=-12\cdot\left(47+52+1\right)=-12\cdot100=-1200\)
7: \(=13\cdot45-3\cdot45=10\cdot45=450\)
8: \(=-48\left(1+78+21\right)=-48\cdot100=-4800\)