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\(4+\frac{1}{x}=\frac{4x+1}{x}\)
\(\frac{1}{4+\frac{1}{x}}=\frac{x}{4x+1}\)
\(3+\frac{1}{4+\frac{1}{x}}=3+\frac{x}{4x+1}=\frac{13x+3}{4x+1}\)
Tương tự Vế Trái sẽ tìm đc
\(21+\frac{12\left(13x+3\right)}{30x+7}\)
Vế phải bấm máy tính nhá casio mà
\(VP=\frac{104052}{137}=21+\frac{101175}{137}\)
Suy ra
\(\frac{156x+36}{30x+7}=\frac{101175}{137}\Leftrightarrow21375x+4932=3035250x+708225\)
\(\Leftrightarrow1004625x=-234431\Leftrightarrow x=-\frac{234431}{1004625}\)
\(\Delta'=m^2-2m+1+m>0\)với mọi m
\(\int^{x1+x2=2\left(m-1\right)}_{x1.x2=-m}\)
\(\int^{y1+y2=\frac{\left(x1+x2\right)\left(x1x2+1\right)}{x1x2}=S}_{y1.y2=.....=P}\Leftrightarrow pt:X^2-SX+P=0\)
Áp dụng BĐT Cauchy Schwarz dạng Engel ta có:
\(\frac{2010}{\sqrt{2011}}+\frac{2011}{\sqrt{2010}}\ge\frac{\left(\sqrt{2010}+\sqrt{2011}\right)^2}{\sqrt{2011}+\sqrt{2010}}=\sqrt{2010}+\sqrt{2011}\left(đpcm\right)\)
:))
A = 22+42+62+...+202
= (1.2)2 + (2.2)2 + (3.2)2 + ... + (10.2)2
= 22 .12 + 22.22 + 22.32 + ... + 22 .102
= 22 . (12 + 22 + 32 + ... + 102)
= 4 . 385
= 1540
Đặt A1 = 1/2^1 + 1/2^2 + ... + 1/2^100
A2 = 1/2^2 + 1/2^3 + ... + 1/2^100
A3 = 1/2^3 + 1/2^4 + ... + 1/2^100
....................................
...................................
A100 = 1/2^100
A = 1/2^1 + 2/2^2 + 3/2^3 + 4/2^4 + ... + 100/2^100 =
= (1/2^1+1/2^2 +...+ 1/2^100) + (1/2^2+1/2^3 +...+ 1/2^100) + (1/2^3+1/2^4 +...+ 1/2^100) + ... + (1/2^100) = A1 + A2 + A3 + ... + A100
2^101 A1 = 2^100 + 2^99 + 2^98 + ... + 2 (1)
2^100 A1 = 2^99 + 2^98 + 2^97 + ... + 1 (2)
(2) trừ (1) ---> 2^100 A1 = 2^100 - 1 ---> A1 = (2^100 - 1) / 2^100 = 1 - 1/2^100
Tương tự
2^101 A2 = 2^99 + 2^98 + 2^97 +...+ 2 (3)
2^100 A2 = 2^98 + 2^97 + 2^96 +...+ 1 (4)
(4) trừ (3) ---> 2^100 A2 = 2^99 - 1 ---> A2 = (2^99 - 1) / 2^100 = 1/2 - 1/2^100
Tương tự
A3 = 1/4 - 1/2^100 = 1/2^2 - 1/2^100
A4 = 1/2^3 - 1/2^100
..................................
.................................
A100 = 1/2^99 - 1/2^100
Vậy A = A1 + A2 + A3 +...+ A100 = (1 + 1/2 + 1/2^2 + ... + 1/2^99) - 100/2^100
= 2 A1 - 100/2^100 = 2 - 2/2^100 - 100/2^100 = 2 - 51/2^99
===========================
Bài 1:
\(A=\sqrt{8}-2\sqrt{2}+\sqrt{20}-2\sqrt{5}-2=2\sqrt{2}-2\sqrt{2}+2\sqrt{5}-2\sqrt{5}-2=-2\)\(B=\frac{\sqrt{x}}{\sqrt{x}-1}-\frac{2\sqrt{x}-1}{\sqrt{x}\left(\sqrt{x}-1\right)}=\frac{x-2\sqrt{x}+1}{\sqrt{x}\left(\sqrt{x}-1\right)}=\frac{\left(\sqrt{x}-1\right)^2}{\sqrt{x}\left(\sqrt{x}-1\right)}=\frac{\sqrt{x}-1}{\sqrt{x}}\)
Lời giải:
Đặt \(A=\frac{1}{\sqrt{1}}+\frac{1}{\sqrt{2}}+....+\frac{1}{\sqrt{2004}}\)
Xét số hạng tổng quát: \(\frac{1}{\sqrt{n}}\) ta có:
\(\frac{1}{\sqrt{n}}=\frac{2}{2\sqrt{n}}> \frac{2}{\sqrt{n}+\sqrt{n+1}}=\frac{2(\sqrt{n+1}-\sqrt{n})}{(\sqrt{n+1}+\sqrt{n})(\sqrt{n+1}-\sqrt{n})}=2(\sqrt{n+1}-\sqrt{n})\)
Do đó:
\(\frac{1}{\sqrt{1}}> 2(\sqrt{2}-\sqrt{1})\)
\(\frac{1}{\sqrt{2}}> 2(\sqrt{3}-\sqrt{2})\)
\(\frac{1}{\sqrt{3}}> 2(\sqrt{4}-\sqrt{3})\)
............
\(\frac{1}{\sqrt{2004}}> 2(\sqrt{2005}-\sqrt{2004})\)
Cộng theo vế:
$A>2(\sqrt{2005}-1)>86$
Vậy..........
