\(A=\left\{n^2\text{ }|\text{ }n\in N,\text{ }1\le n\le7\text{ }\right\}\)

Hoặc:

\(A=\left\{x\text{ }|\text{ }\text{x là số chính phương},\text{ }0< x< 50\right\}\) 

\(\left(x-2\right)^2-\left(x+3\right)^2+\left(x+4\right)\left(x-4\right)=0\\ < =>x^2-4x+4-x^2-6x-9+x^2-16=0\\ < =>x^2-10x-21=0\\ < =>\left(x^2-10x+25\right)-46=0\\ < =>\left(x-5\right)^2=46\\ < =>\left[{}\begin{matrix}x-5=\sqrt{46}\\x-5=-\sqrt{46}\end{matrix}\right.\\ < =>\left[{}\begin{matrix}x=\sqrt{46}+5\\x=5-\sqrt{46}\end{matrix}\right.\)

\(\dfrac{3}{5}+x=5-\dfrac{1}{2}\\ \dfrac{3}{5}+x=\dfrac{10}{2}-\dfrac{1}{2}\\ \dfrac{3}{5}+x=\dfrac{10-1}{2}\\ \dfrac{3}{5}+x=\dfrac{9}{2}\\ x=\dfrac{9}{2}-\dfrac{3}{5}\\ x=\dfrac{45}{20}-\dfrac{6}{10}\\ x=\dfrac{45-6}{10}\\ x=\dfrac{39}{10}\)

Vậy: ... 

\(7-\left(x-1\right)=15+3\left(x+1\right)\\ 7-x+1=15+3x+3\\ 8-x=18+3x\\ 3x+x=8-18\\ 4x=-10\\ x=-\dfrac{10}{4}\\ x=\dfrac{-5}{2}\)

Vậy: ... 

\(\left(\dfrac{-5}{9}\right)^{10}:x=\left(\dfrac{-5}{9}\right)^8\\ =>x=\left(\dfrac{-5}{9}\right)^{10}:\left(\dfrac{-5}{9}\right)^8\\ =>x=\left(-\dfrac{5}{9}\right)^{10-8}\\ =>x=\left(-\dfrac{5}{9}\right)^2\\ =>x=\dfrac{\left(-5\right)^2}{9^2}\\ =>x=\dfrac{25}{81}\)

\(\left(-\dfrac{5}{9}\right)^{10}:x=\left(-\dfrac{5}{9}\right)^8\\ \Rightarrow x=\left(-\dfrac{5}{9}\right)^{10-8}\\ \Rightarrow x=\left(-\dfrac{5}{9}\right)^2\\ \Rightarrow x=\dfrac{25}{81}\)

Vậy: \(x=\dfrac{25}{81}\)

\(\left|4-x\right|+2x=0\)

`TH1:x<=4`

`(4-x)+2x=0`

`=>4-x+2x=0`

`=>x+4=0`

`=>x=-4(tm)` 

`TH2:x>4`

`-(4-x)+2x=0`

`=>-4+x+2x=0`

`=>3x-4=0`

`=>3x=4`

`=>x=4/3` (ktm)

Vậy: ... 

\(x^2+2\left|x-\dfrac{1}{2}\right|=\left|x^2+2\right|\\ =>x^2+2\left|x-\dfrac{1}{2}\right|=x^2+2\left(x^2+2>0\right)\\ =>2\left|x-\dfrac{1}{2}\right|=2\\ =>\left|x-\dfrac{1}{2}\right|=1\\ TH1:x>\dfrac{1}{2}\\ =>x-\dfrac{1}{2}=1\\ =>x=1+\dfrac{1}{2}=\dfrac{3}{2}\\ TH2:x< =\dfrac{1}{2}\\ =>x-\dfrac{1}{2}=-1\\ =>x=\dfrac{1}{2}-1=-\dfrac{1}{2}\left(tm\right)\)

no

a.

\(a+b+c=0\Rightarrow\left(a+b+c\right)^2=0\)

\(\Rightarrow a^2+b^2+c^2=-2\left(ab+bc+ca\right)\)

\(\Rightarrow\left(a^2+b^2+c^2\right)^2=4\left(ab+bc+ca\right)^2\)

\(\Rightarrow a^4+b^4+c^4+2\left(a^2b^2+b^2c^2+c^2a^2\right)=4\left(a^2b^2+b^2c^2+c^2a^2\right)+8abc\left(a+b+c\right)\)

\(\Rightarrow a^4+b^4+c^4=2\left(a^2b^2+b^2c^2+c^2a^2\right)\)

b.

Từ câu a:

\(a^4+b^4+c^4=2\left(a^2b^2+b^2c^2+c^2a^2\right)\)

\(\Rightarrow2\left(a^4+b^4+c^4\right)=a^4+b^4+c^4+2\left(a^2b^2+b^2c^2+c^2a^2\right)\)

\(\Rightarrow2\left(a^4+b^4+c^4\right)=\left(a^2+b^2+c^2\right)^2\)

\(\Rightarrow a^4+b^4+c^4=\dfrac{\left(a^2+b^2+c^2\right)^2}{2}\)

                Giải:

Tỉ số số bi của An và số bi của Bình là:

          \(\dfrac{1}{7}\) : \(\dfrac{1}{2}\) = \(\dfrac{2}{7}\) 

Theo bài ra ta có sơ đồ:

Theo sơ đồ ta có:

Số bi của An là:

150 : (7 - 2) x 2 = 60 (viên bi)

Số bi của Bình là:

150 +  60 =  210 (viên bi)

Đáp số: .... 

\(\left(1-\dfrac{1}{2}\right)\times\left(1-\dfrac{1}{3}\right)\times\left(1-\dfrac{1}{4}\right)\times\left(1-\dfrac{1}{5}\right)\\ =\dfrac{1}{2}\times\dfrac{2}{3}\times\dfrac{3}{4}\times\dfrac{4}{5}\\ =\dfrac{1}{5}\\ ===============\\ \left(1-\dfrac{3}{4}\right)\times\left(1-\dfrac{3}{7}\right)\times\left(1-\dfrac{3}{10}\right)\times...\times\left(1-\dfrac{3}{97}\right)\times\left(1-\dfrac{3}{100}\right)\\ =\dfrac{1}{4}\times\dfrac{4}{7}\times\dfrac{7}{10}\times...\times\dfrac{94}{97}\times\dfrac{97}{100}\\ =\dfrac{1}{100}\)

\(\left(1-\dfrac{1}{2}\right)\times\left(1-\dfrac{1}{3}\right)\times\left(1-\dfrac{1}{4}\right)\times\left(1-\dfrac{1}{5}\right)\)

\(=\dfrac{1}{2}\times\dfrac{2}{3}\times\dfrac{3}{4}\times\dfrac{4}{5}\)

\(=\dfrac{1\times2\times3\times4}{2\times3\times4\times5}\)

\(=\dfrac{1}{5}\)

\(\left(1-\dfrac{3}{4}\right)\times\left(1-\dfrac{3}{7}\right)\times\left(1-\dfrac{3}{10}\right)\times\left(1-\dfrac{3}{13}\right)\times...\times\left(1-\dfrac{3}{97}\right)\times\left(1-\dfrac{3}{100}\right)\)

\(=\dfrac{1}{4}\times\dfrac{4}{7}\times\dfrac{7}{10}\times\dfrac{10}{13}\times...\times\dfrac{94}{97}\times\dfrac{97}{100}\)

\(=\dfrac{1\times4\times7\times10\times...\times94\times97}{4\times7\times10\times13\times...\times97\times100}\)

\(=\dfrac{1}{100}\)