tinh nhanh
1x2+2x3+3x4+4x5+......+9x10=?
tinh 1/2x3+1/3x4+1/4x5+1/5x6+1/6x7+1/7x8+1/8x9+1/9x10=
Tra loi:
1/2x3+1/3x4+1/4x5+1/5x6+1/6x7+1/7x8+1/8x9
=1/2 - 1/3 + 1/3 - 1/4 +1/4 - 1/5 + 1/5 -1/6 +1/6 - 1/7 +1/7 -1/8+1/8 - 1/9
=1/2-(1/3-1/3)-(1/4-1/4)-(1/5-1/5)-(1/6-1/6)-(1/7-1/7)-(1/8-1/8)-1/9
=1/2-0-0-0-0-0-0-1/9
=1/2-1/9
=7/18
k cho minh nha!
thông điệp nhỏ
hai tích nếu như ko muốn k
ai tích mình tích lại
1/2x3 + 1/3x4 + 1/4x5 + ... + 1/9x10 = ?
1/2-1/3+1/3-1/4+....+1/9-1/10
Ta có 1/2-1/10=2/5
1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/ 6x7 + 1/7x8 + 1/8x9 + 1/9x10
= 2/5
1/2x3 + 1/3x4 + 1/4x5...+ 1/9x10
bạn theo công thức nay nè
\(\frac{n}{a\left(a+n\right)}=\frac{1}{a}-\frac{1}{a+n}\)là giải đc thôi
\(\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+...+\frac{1}{9x10}\) \(=\) \(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{2}-\frac{1}{10}\)
\(=\frac{2}{5}\)
1/2x3 + 1/3x4 +1/4x5 +...+ 1/9x10
=1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 +...+ 1/9 - 1/10
=1/2 - 1/10
=5/10 - 1/10
=4/10
=2/5
1/ 2x3+ 1/ 3x4+ 1/ 4x5.....+ 1/ 9x10 =
\(=\frac{1}{2}-\frac{1}{10}=\frac{2}{5}\)
=1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10 =1/2-1/10 =2/5
\(\Leftrightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
\(\Rightarrow\frac{1}{2}-\frac{1}{10}=\frac{5}{10}-\frac{1}{10}=\frac{4}{10}=\frac{2}{5}\)
2021/1x2+2021/2x3+2021/3x4+2021/4x5+...+2021/9x10
\(\frac{2021}{1\cdot2}+\frac{2021}{2\cdot3}+...+\frac{2021}{9\cdot10}=2021\cdot\left(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{9\cdot10}\right)\)
\(=2021\cdot\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\right)=2021\cdot\left(1-\frac{1}{10}\right)\)
\(=2021\cdot\frac{9}{10}=\frac{18189}{10}\)
Ta có : \(\frac{2021}{1.2}+\frac{2021}{2.3}+\frac{2021}{3.4}+...+\frac{2021}{9.10}=2021\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\right)\)
\(=2021\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\right)\)
\(=2021\left(1-\frac{1}{10}\right)=2021.\frac{9}{10}=1818,9\)
Tính:
1/2x3 + 1/3x4+ 1/4x5+ ......+1/9x10
\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)
\(=\frac{3-2}{2.3}+\frac{4-3}{3.4}+\frac{5-4}{4.5}+...+\frac{10-9}{9.10}\)
\(=\frac{3}{2.3}-\frac{2}{2.3}+\frac{4}{3.4}-\frac{3}{3.4}+...+\frac{10}{9.10}-\frac{9}{9.10}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{2}-\frac{1}{10}=\frac{2}{5}\)
tính nhanh
1x2 + 2x3 + 3x4 + 4x5 +5x6 + 6x7 + 7x8 + 8x9 + 9x10
1/2x3 +1/3x4 + 1/4x5 +....+ 1/9x10
có lời giải nhé
1/2.3+1/3.4+...+1/9.10
=1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10
=1/2-1/10=2/5
Ta có : \(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+......+\frac{1}{9.10}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.......+\frac{1}{10}\)
\(=\frac{1}{2}-\frac{1}{10}=\frac{5}{10}-\frac{1}{10}=\frac{2}{5}\)
jup toi giai bai nay voi 1x2+2x3+3x4+4x5+......+9x10
\(1.2+2.3+3.4+4.5+...+9.10\)
Đặt A = \(1.2+2.3+3.4+4.5+...+9.10\)
\(3.A\) = \(1.2.3+2.3.3+3.4.3+4.5.3+...+9.10.3\)
=> \(3.A\) = \(1.2.3+2.3.4+3.4.5+4.5.6+...+9.10.11\)
Ta có: \(3.A=9.10.11\)
=>: \(A=\frac{9.10.11}{3}=330\)
=> A = 330
\(1.2+2.3+4.5+...+9.10\)
Dat A=1.2+2.3+3.4+4.5+...+9.10
3.A=1.2.3+2.3.3+3.4.3+4.5.3+...+9.10.3
3A=1.2.3+2.3.4+3.4.5+4.5.6+...+9.10.11
Ta co:3A=9.10.11
A=9.10.11:3
A=330
Vay A=330