tính hợp lí
a)\(\frac{45x16-17}{45x15-28}\)
b)\(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^8}\)
c)136.68+16x272
K
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NK
a,\(\frac{45x16-17}{45x15+28}\) b,5\(\frac{3}{4}\)x2\(\frac{1}{2}\)+\(\frac{7}{2}\):\(1\frac{1}{4}\)
1
25 tháng 9 2017
a) \(\frac{45x16-17}{45x15+28}=\frac{45x\left(15+1\right)-17}{45x15+28}\)\(=\frac{45x15+45-17}{45x15+28}=\frac{45x15+28}{45x15+28}=1\)
b) \(5\frac{3}{4}x2\frac{1}{2}+\frac{7}{2}:1\frac{1}{4}=\frac{23}{4}x\frac{5}{2}+\frac{7}{2}:\frac{5}{4}=\frac{115}{8}+\frac{7}{2}x\frac{4}{5}=\frac{115}{8}+\frac{14}{5}=\frac{575+112}{40}=\frac{678}{40}\)
14 tháng 7 2018
\(17,58\times43+5,7\times175,8\)
\(=17,58\times43+5,7\times10\times17,58\)
\(=17,58\times43+57\times17,58\)
\(=17,58\times\left(43+57\right)\)
\(=17,58\times100\)
\(=1758\)
_Chúc bạn học tốt_
14 tháng 7 2018
\(17,58\times43+5,7\times175,8=17,58\times10\times4,3+5,7\times175,8\)
\(=175,8\times4,3+5,7\times175,8\)
\(=175,8\times\left(4,3+5,7\right)=175,8\times10=1758\)
1 tháng 8 2016
\(\frac{45.16-17}{45.15+28}=\frac{45.15+45-17}{45.15+28}=\frac{45.15+28}{45.15+28}=1\)
1 tháng 8 2016
45 × 16 - 17/45 × 15 + 28
= 45 × (15 + 1) - 17/45 × 15 + 28
= 45 × 15 + (45 - 17)/45 × 15 + 28
= 45 × 15 + 28/45 × 15 + 28
= 1
MH
18 tháng 4 2016
\(\frac{45.16-17}{45.15+28}=\frac{45.\left(15+1\right)-17}{45.15+28}=\frac{45.15+45-17}{45.15+28}=\frac{45.15+28}{45.15+28}=1\)
22 tháng 6 2017
Đặt A = \(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\)
3A = \(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\)
3A - A = (\(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\)) - (\(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\))
2A = 1 - \(\frac{1}{729}\) = \(\frac{728}{729}\)
A = \(\frac{728}{729}:2=\frac{364}{729}\)
Đặt \(A=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^8}\)
\(\Leftrightarrow\)\(3A=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^7}\)
\(\Leftrightarrow\)\(3A-A=\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^7}\right)-\left(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^8}\right)\)
\(\Leftrightarrow\)\(2A=1-\frac{1}{3^8}\)
\(\Leftrightarrow\)\(2A=\frac{3^8-1}{3^8}\)
\(\Leftrightarrow\)\(A=\frac{3^8-1}{3^8}:2\)
\(\Leftrightarrow\)\(A=\frac{3^8-1}{3^8}.\frac{1}{2}\)
\(\Leftrightarrow\)\(A=\frac{3^8-1}{2.3^8}\)
b) Đặt \(A=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+.....+\frac{1}{3^8}\)
\(\Rightarrow\)\(3A=1+\frac{1}{3}+\frac{1}{3^2}+....+\frac{1}{3^7}\)
\(\Rightarrow\)\(3A-A=\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^7}\right)-\left(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+....+\frac{1}{3^8}\right)\)
\(\Rightarrow\)\(2A=1-\frac{1}{3^8}\)
\(\Rightarrow\)\(A=\frac{1-\frac{1}{3^8}}{2}\)