`15xx(x-7)-25xx4+17=37`

`15xx(x-7)-100+17=37`

`15xx(x-7)-100=37-17`

`15xx(x-7)-100=20`

`15xx(x-7)=20+100`

`15xx(x-7)=120`

`x-7=120:15`

`x-7=8`

`x=8+7`

`x=15`

Bài `1:`

`a)` Các số trên đều `in QQ` vì chúng đều có thể viết dưới dạng p/s

`b)`

\(-3=3\\ 4\dfrac{1}{5}=\dfrac{21}{5}=-\dfrac{21}{5}\\ 2,5=-2,5\\ 13=-13\\ 0=0\\ -4\dfrac{1}{3}=-\dfrac{13}{3}=\dfrac{13}{3}\)

đọc kĩ đề nhé!

`x vdots 5;13`

`=>x in BC(5;13). ĐK:13<=x<=78`

`=>BC(5;13)=B(65)={0;65;130;...}`

`=>x=65`

\(A=\dfrac{10^{11}-1}{10^{12}-1}\\ 10A=\dfrac{10^{12}-10}{10^{12}-1}\\ 10A=1-\dfrac{9}{10^{12}-1}\\ \)

\(B=\dfrac{10^{10}+1}{10^{11}+1}\\ 10B=\dfrac{10^{11}+10}{10^{11}+1}\\ 10B=1+\dfrac{9}{10^{11}+1}\)

Vì `10^(11)+1<10^(12)-1` nên \(\dfrac{9}{10^{11}+1}>\dfrac{9}{10^{12}-1}\\ \Rightarrow1+\dfrac{9}{10^{11}+1}>1-\dfrac{9}{10^{12}-1}\\ \Rightarrow10B>10A\\ \Rightarrow B>A\)

\(a,\\ 27+8y^3\\ =\left(2y+3\right)\left(4y^2-6y+9\right)\\ b,\\ \dfrac{1}{8}x^3-1\\ =\dfrac{1}{8}\left(x-2\right)\left(x^2+2x+4\right)\\ c,\\ -x^3-\dfrac{1}{27}\\ =-\dfrac{1}{27}\left(27x^3+1\right)\\ =-\dfrac{1}{27}\left(3x+1\right)\left(9x^2-3x+1\right)\)

`x+3,25xx8,2=38,89`

`x+26,65=38,89`

`x=38,89-26,65`

`x=12,24`

\(\left(x-7,025\right)\times2,5=27,75\\ x-7,025=27,75:2,5\\ x-7,025=11,1\\ x=11,1+7,025\\ x=18,125\\ ~~~~~~~~~~~\\ x-18,5\times2,2=88,88\\ x-40,7=88,88\\ x=88,88+40,7\\ x=129,58\)

\(-\dfrac{5}{x}=-\dfrac{18}{72}\\ \Leftrightarrow-18x=-360\\ \Leftrightarrow x=20\\ \dfrac{-5}{20}=\dfrac{y}{16}\\ \Leftrightarrow\dfrac{-1}{4}=\dfrac{y}{16}\\ \Leftrightarrow4y=-16\\ \Leftrightarrow y=-4\)