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Số số hạng của dãy là : (397−1):4+1=100(397-1):4+1=100 ( số )
⇒⇒ Có 100:2=50100:2=50 ( cặp )
1−5+9−13+....+393−3971-5+9-13+....+393-397
=(1−5)+(9−13)+....+(393−397)=(1-5)+(9-13)+....+(393-397)
=(−4)+(−4)+....+(−4)=(-4)+(-4)+....+(-4)
=(−4).50=−200=(-4).50=-200
TK`1-5+9-13+...+393-397`
`=(1-5)+(9-13)+...+(393-397)` (197 cặp)
`=(-4)+(-4)+...+(-4)`
`=(-4) xx 197`
`=-788`
\(\left(13\frac{4}{9}+2\frac{1}{9}\right)-3\frac{4}{9}\)
\(=\left(\frac{121}{9}+\frac{19}{9}\right)-\frac{31}{9}\)
\(=\frac{140}{9}-\frac{31}{9}\)
\(=\frac{109}{9}\)
\(1,25\div\frac{15}{20}+\left(25\%-\frac{5}{6}\right)\div4\frac{2}{3}\)
\(=1,25\div\frac{3}{4}+\left(\frac{25}{100}-\frac{5}{6}\right)\div\frac{14}{3}\)
\(=1,25\times\frac{4}{3}+\left(\frac{6}{24}-\frac{20}{24}\right)\div\frac{14}{3}\)
\(=\frac{5}{3}+\frac{-14}{24}\times\frac{14}{3}\)
\(=\frac{60}{36}+\frac{-98}{36}\)
\(=\frac{-38}{36}=\frac{-19}{18}\)
a,\(\frac{7}{10}\cdot\frac{4}{9}+\frac{3}{10}\cdot\frac{4}{9}-1\frac{7}{9}\)
\(=\frac{14}{45}+\frac{2}{15}-\frac{16}{9}\)
\(=\frac{14}{45}+\frac{6}{45}-\frac{80}{45}\)
\(=\frac{-60}{45}=\frac{-4}{3}\)
b,\(\frac{-5}{6}+\frac{4}{9}\cdot\left(\frac{5}{4}-\frac{2}{3}\right)\cdot\left(-3\right)^2+\frac{5}{9}\cdot30\%\)
\(=\frac{-5}{6}+\frac{4}{9}\cdot\left(\frac{7}{12}\right)\cdot9+\frac{5}{9}\cdot\frac{3}{10}\)
\(=\frac{-5}{6}+\frac{7}{3}+\frac{1}{6}\)
\(=\frac{-5}{6}+\frac{14}{6}+\frac{1}{6}\)
=\(=\frac{10}{6}=\frac{5}{3}\)
a, ( - 7 ) x ( - 8 ) = 56
b, 3 / 7 x ( - 2 / 9 ) = - 6 / 63 = - 2 / 21
c, - 7 / 6 : 7 / 12 = - 7 / 6 x 12 / 7 = - 12 / 6 = -2
d, - 1 / 2 x 3 / 4 + 1 / 4 x ( - 1 / 2 ) = - 1 / 2 x ( 3 / 4 + 1 / 4 ) = - 1 / 2 x 1 = - 1 / 2
Bài 2:
Ta thấy: 52 > 4.5
62 > 5.6
72 > 6.7
....
20172 > 2016.2017
\(\Rightarrow\)\(\frac{1}{5^2}< \frac{1}{4.5}\)
\(\frac{1}{6^2}< \frac{1}{5.6}\)
\(\frac{1}{7^2}< \frac{1}{6.7}\)
....
\(\frac{1}{2017^2}< \frac{1}{2016.2017}\)
Cộng vế với nhau, ta có:
\(\frac{1}{5^2}+\frac{1}{6^2}+\frac{1}{7^2}+...+\frac{1}{2017^2}\) < \(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{2016.2017}\)
\(\Rightarrow\)A < \(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{2016}-\frac{1}{2017}\)
\(\Rightarrow\)A < \(\frac{1}{4}-\frac{1}{2017}\)
\(\Rightarrow\)A < \(\frac{1}{4}\)( vì \(\frac{1}{2017}>0\))
k giúp mik ✅
Bài 1 :
36/1212 = 3/101
13/1313 = 1/101
3/101 + 1/101 = 4/101
Vậy 36/1212 + 13/1313 = 4/101.
Bài 2 :
A = 5/13 + 1/2 + -5/9 + -3/6 + 4/-9
A = 5/13 + 1/2 + -5/9 + -1/2 + -4/9
A = (1/2 + -1/2) + (-5/9 + -4/9) + 5/13
A = 0 + (-1) + 5/13
A = (-1) + 5/13 = -13/13 + 5/13 = 8/13.
Chúc bạn học giỏi nhé.
a) 1 - 5 + 9 - 13 + .... + 393 - 397
= ( 1 - 5 ) + ( 9 - 13 ) + ... + ( 393 - 397 )
= ( -4 ) + ( -4 ) + ... + ( -4 )
= ( -4 ) x 197
= -788
b) - 2 + 4 - 6 + 8 - ... - 238 + 240
= ( -2 + 4 ) + ( -6 + 8 ) + ... + ( -238 + 240 )
= 2 + 2 + ... + 2
= 2 x 60
= 140
Cảm ơn nha!!!