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Ta có:
b2=a.c c2=b.d
\(\Rightarrow\frac{b}{c}=\frac{a}{b}\) \(\Rightarrow\frac{b}{c}=\frac{c}{d}\)
\(\Rightarrow\frac{a}{b}=\frac{b}{c}=\frac{c}{d}\) (1)
\(\Rightarrow\hept{\begin{cases}\left(1\right)=\frac{a^{2017}}{b^{2017}}=\frac{b^{2017}}{c^{2017}}=\frac{c^{2017}}{d^{2017}}=\frac{a^{2017}+b^{2017}-c^{2017}}{b^{2017}+c^{2017}d^{2017}}\\\left(1\right)=\frac{a+b-c}{b+c-d}=\frac{\left(a+b-c\right)^{2017}}{\left(b+c-d\right)^{2017}}\end{cases}}\)
\(\Rightarrow\frac{a^{2017}+b^{2017}-c^{2017}}{b^{2017}+c^{2017}d^{2017}}=\frac{\left(a+b-c\right)^{2017}}{\left(b+c-d\right)^{2017}}\)
Vậy \(\frac{a^{2017}+b^{2017}-c^{2017}}{b^{2017}+c^{2017}d^{2017}}=\frac{\left(a+b-c\right)^{2017}}{\left(b+c-d\right)^{2017}}\)
Ta có: \(b^2=a\cdot c\Rightarrow\frac{a}{b}=\frac{b}{c}\left(1\right)\)
\(c^2=b\cdot d\Rightarrow\frac{b}{c}=\frac{c}{d}\left(2\right)\)
Từ (1) và (2) \(\Rightarrow\frac{a}{b}=\frac{b}{c}=\frac{c}{d}\)
\(\Rightarrow\frac{a^{2017}}{b^{2017}}=\frac{b^{2017}}{c^{2017}}=\frac{c^{2017}}{d^{2017}}\)
Áp dụng tính chất dãy tỉ số bằng nhau, ta có:
\(\frac{a^{2017}}{b^{2017}}=\frac{b^{2017}}{c^{2017}}=\frac{c^{2017}}{d^{2017}}=\frac{a^{2017}+b^{2017}-c^{2017}}{b^{2017}+c^{2017}-d^{2017}}\)(3)
Ta có: \(\frac{a}{b}=\frac{b}{c}=\frac{c}{d}=\frac{a+b-c}{b+c-d}\)
\(\Rightarrow\frac{a^{2017}}{b^{2017}}=\frac{\left(a+b-c\right)^{2017}}{\left(b+c-d\right)^{2017}}\)(4)
Từ (3) và (4) \(\Rightarrow\frac{a^{2017}+b^{2017}-c^{2017}}{b^{2017}+c^{2017}-d^{2017}}=\frac{\left(a+b-c\right)^{2017}}{\left(b+c-d\right)^{2017}}\)(đpcm)
=>\(-B=\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)...\left(1-\frac{1}{2012}\right)\)
=\(\frac{1}{2}.\frac{2}{3}...\frac{2011}{2012}=\frac{1}{2012}\)
Bài 1:
\(A=\frac{a+b}{b+c}.\)
Ta có:
\(\frac{b}{a}=2\Rightarrow\frac{b}{2}=\frac{a}{1}\) (1)
\(\frac{c}{b}=3\Rightarrow\frac{c}{3}=\frac{b}{1}\) (2)
Từ (1) và (2) \(\Rightarrow\frac{b}{2}=\frac{c}{6}.\)
\(\Rightarrow\frac{a}{1}=\frac{b}{2}=\frac{c}{6}=\frac{a+b}{3}=\frac{b+c}{8}.\)
\(\Rightarrow A=\frac{a+b}{b+c}=\frac{3}{8}\)
Vậy \(A=\frac{a+b}{b+c}=\frac{3}{8}.\)
Bài 2:
a) \(\frac{72-x}{7}=\frac{x-40}{9}\)
\(\Rightarrow\left(72-x\right).9=\left(x-40\right).7\)
\(\Rightarrow648-9x=7x-280\)
\(\Rightarrow648+280=7x+9x\)
\(\Rightarrow928=16x\)
\(\Rightarrow x=928:16\)
\(\Rightarrow x=58\)
Vậy \(x=58.\)
b) \(\frac{x+4}{20}=\frac{5}{x+4}\)
\(\Rightarrow\left(x+4\right).\left(x+4\right)=5.20\)
\(\Rightarrow\left(x+4\right).\left(x+4\right)=100\)
\(\Rightarrow\left(x+4\right)^2=100\)
\(\Rightarrow x+4=\pm10.\)
\(\Rightarrow\left[{}\begin{matrix}x+4=10\\x+4=-10\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=10-4\\x=\left(-10\right)-4\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=6\\x=-14\end{matrix}\right.\)
Vậy \(x\in\left\{6;-14\right\}.\)
Chúc bạn học tốt!
Bài 2:
a, \(\frac{72-x}{7}=\frac{x-40}{9}\)
\(\Rightarrow\left(72-x\right).9=\left(x-40\right).7\)
\(\Rightarrow9.72-9.x=7.x-7.40\)
\(\Rightarrow648-9x=7x-280\)
\(\Rightarrow-9x-7x=-280-648\)
\(\Rightarrow-16x=-648\)
\(\Rightarrow x=58\)
Vậy \(x=58\)
Ta có:
\(\frac{x}{x+1}=1-\frac{1}{x+1}\in Z\Rightarrow x+1\inƯ\left(1\right)\Rightarrow x+1\in\left\{-1;1\right\}\Rightarrow x\in\left\{-2;0\right\}\)
\(+,x=0;\Rightarrow\frac{x}{x+1}=0\left(tm\right);+,x=-2\Rightarrow\frac{x}{x+1}=\frac{-2}{-1}=2\left(tm\right)\)
Vậy: x E {0;2}
b, \(\frac{a}{2010}=\frac{b}{2012}=\frac{c}{2014}\Rightarrow a=2010k;b=2012k;c=2014k\left(k\in Z\right)\)
\(\frac{\left(a-c\right)^2}{4}=\frac{\left(-4k\right)^2}{4}=\frac{16k^2}{4}=4k^2\)và: \(\left(a-b\right)\left(b-c\right)=\left(-2k\right)\left(-2k\right)=4k^2\)
\(\frac{\left(a-c\right)^2}{4}=\left(a-b\right)\left(b-c\right)\)\(\left(ĐPCM\right)\)
c, Ta có:
\(25-y^2=8.x^2\Rightarrow25-y^2⋮8\Rightarrow y^2:8\left(dư1\right)\left(y\le5\right)\Rightarrow y\in\left\{1;3;5\right\}\)
Ta lần lượt thử ta thấy:
\(25-y^2=8.x^2\left(tm\right)\Leftrightarrow y=5\Rightarrow x=0\)
Vậy: y=5;x=0
a) \(4.\left(\frac{-1}{3}\right)^3-6.\left(\frac{-1}{2}\right)^2+\left(\frac{1}{2}\right)^0\)
= \(4.\left(\frac{-1}{27}\right)-6.\left(\frac{1}{4}\right)+1\)
= \(\frac{-4}{27}-\frac{3}{2}+1\)
= \(\frac{-8}{54}-\frac{81}{54}+\frac{54}{54}\)
= \(\frac{-35}{54}\)
b) \(\left(3^2\right)^2-\left[\left(-5\right)^2\right]^2+\left[\left(-2\right)^3\right]^2\)
= \(3^{2.2}-\left(-5\right)^{2.2}+\left(-2\right)^{3.2}\)
= \(3^4-\left(-5\right)^4+\left(-2\right)^6\)
= \(81-625+64\)
= \(-480\)