S=1x3+2x4+3x5+...........+99x101
1x3/3x5 + 2x4/5x7 + 3x5/7x9 + ............... +49x51/99x101
1x3+2x4+3x5+4x6+.......+99x101+100x102
=1(2+1)+2(3+1)+3(4+1)+...+100(101+1)
=1.2+1+2.3+2+3.4+3+...+100.101+100
=(1.2+2.3+3.4+..+100.101)+(1+2+3+...+100)
=333300+5000
=338300
tính : 1x3+2x4+3x5+.....+99x101+100x102
tính : 1x3 + 2x4 + 3x5 + 4x6 + ... + 99x101 . Tính giúp mik nhé :))))
khó dữ vậy ba ?????
Tìm số tự nhiên K biết:
K=1x3+2x4+3x5+.............+99x101
A=(1+1/1x3)x(1+1/2x4)x(1+1/3x5)x...x(1+1/99x101)
Lời giải:
Xét thừa số tổng quát $1+\frac{1}{n(n+2)}=\frac{n(n+2)+1}{n(n+2)}=\frac{(n+1)^2}{n(n+2)}$
Khi đó:
$1+\frac{1}{1.3}=\frac{2^2}{1.3}$
$1+\frac{1}{2.4}=\frac{3^2}{2.4}$
.........
$1+\frac{1}{99.101}=\frac{100^2}{99.101}$
Khi đó:
$A=\frac{2^2.3^2.4^2......100^2}{(1.3).(2.4).(3.5)....(99.101)}$
$=\frac{(2.3.4...100)(2.3.4...100)}{(1.2.3...99)(3.4.5...101)}$
$=\frac{2.3.4...100}{1.2.3..99}.\frac{2.3.4...100}{3.4.5..101}$
$=100.\frac{2}{101}=\frac{200}{101}$
(1+1/1x3)x(1+1/2x4)x(1+1/3x5)x...x(1+1/99x101)
\(\left(1+\frac{1}{1\times3}\right)\times\left(1+\frac{1}{2\times4}\right)\times\left(1+\frac{1}{3\times5}\right)\times...\times\left(1+\frac{1}{99.101}\right)\)
\(=\left(\frac{3}{3}+\frac{1}{3}\right)\times\left(\frac{8}{8}+\frac{1}{8}\right)\times\left(\frac{15}{15}+\frac{1}{15}\right)\times...\times\left(\frac{9999}{9999}+\frac{1}{9999}\right)\)
\(=\frac{4}{3}\times\frac{9}{8}\times\frac{16}{15}\times...\times\frac{10000}{9999}\)
\(=\frac{4\times9\times16\times...\times10000}{3\times8\times15\times...\times9999}\)
\(=\frac{2\times2\times3\times3\times4\times4\times...\times100\times100}{1\times3\times2\times4\times3\times5\times...\times99\times101}\)
\(=\frac{2\times100}{101}=\frac{200}{101}\)
Tính:
A=1x3+2x4+3x5+...+99x101
(1+1/1x3) x (1+1/2x4) x (1+1/3x5) x...x (1+1/99x101) x X = 100/101