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Question Number 10323 by amir last updated on 04/Feb/17

Answered by mrW1 last updated on 04/Feb/17

there are 4 circles which tangent  the curve and the both coordinate axes.  they tangent the curve at point (1,1)  as well as at point(−1,−1).    let r be the radius of the circles.  (r−1)^2 +(r−1)^2 =r^2   2r^2 −4r+2=r^2   r^2 −4r+2=0  r=((4±(√(4^2 −4×2)))/2)=2±(√2)  r_1 =2−(√2)≈0.585  r_2 =2+(√2)≈3.414    r_1  is the radius of the small circles.  r_2  is the redius of the big circles.

$${there}\:{are}\:\mathrm{4}\:{circles}\:{which}\:{tangent} \\ $$$${the}\:{curve}\:{and}\:{the}\:{both}\:{coordinate}\:{axes}. \\ $$$${they}\:{tangent}\:{the}\:{curve}\:{at}\:{point}\:\left(\mathrm{1},\mathrm{1}\right) \\ $$$${as}\:{well}\:{as}\:{at}\:{point}\left(−\mathrm{1},−\mathrm{1}\right). \\ $$$$ \\ $$$${let}\:{r}\:{be}\:{the}\:{radius}\:{of}\:{the}\:{circles}. \\ $$$$\left({r}−\mathrm{1}\right)^{\mathrm{2}} +\left({r}−\mathrm{1}\right)^{\mathrm{2}} ={r}^{\mathrm{2}} \\ $$$$\mathrm{2}{r}^{\mathrm{2}} −\mathrm{4}{r}+\mathrm{2}={r}^{\mathrm{2}} \\ $$$${r}^{\mathrm{2}} −\mathrm{4}{r}+\mathrm{2}=\mathrm{0} \\ $$$${r}=\frac{\mathrm{4}\pm\sqrt{\mathrm{4}^{\mathrm{2}} −\mathrm{4}×\mathrm{2}}}{\mathrm{2}}=\mathrm{2}\pm\sqrt{\mathrm{2}} \\ $$$${r}_{\mathrm{1}} =\mathrm{2}−\sqrt{\mathrm{2}}\approx\mathrm{0}.\mathrm{585} \\ $$$${r}_{\mathrm{2}} =\mathrm{2}+\sqrt{\mathrm{2}}\approx\mathrm{3}.\mathrm{414} \\ $$$$ \\ $$$${r}_{\mathrm{1}} \:{is}\:{the}\:{radius}\:{of}\:{the}\:{small}\:{circles}. \\ $$$${r}_{\mathrm{2}} \:{is}\:{the}\:{redius}\:{of}\:{the}\:{big}\:{circles}. \\ $$

Commented by mrW1 last updated on 04/Feb/17

Commented by amir last updated on 08/Feb/17

hello dear mrw1.thank you very much  I have another solotion for this Q.  because of the circle tangent to the   cordinate axes ,the cordinates of   its center are O ((r),(r) )  and its equation  will be (x−r)^2 +(y−r)^2 =r^(2.)   by replacing  y=(1/x) in this equation  we have   (x−r)^2 +((1/x)−r)^2 =r^2   x^2 (x−r)^2 +(1−rx)^2 =r^2 x^2   x^4 −2rx^3 +r^2 x^2 −2rx+1=0  or  x^2 r^2 −2x(x^2 +1)r+(1+x^4 )=0  r=((−b±(√(b^2 −4ac)))/(2a))=((−2x(x^2 +1)±(√(4x^2 (x^2 +1)^2 −4x^2 (1+x^4 ))))/(2x^2 ))  r=x+(1/x)±(√2)  p=2πr=2π(x+(1/x)±(√2))  (dp/dx)=0   2π(1−(1/x^2 ))=0  x^2 −1=0       x=1    x=−1  r=1+(1/1)±(√(2=))2±(√2)    ■

$${hello}\:{dear}\:{mrw}\mathrm{1}.{thank}\:{you}\:{very}\:{much} \\ $$$${I}\:{have}\:{another}\:{solotion}\:{for}\:{this}\:{Q}. \\ $$$${because}\:{of}\:{the}\:{circle}\:{tangent}\:{to}\:{the}\: \\ $$$${cordinate}\:{axes}\:,{the}\:{cordinates}\:{of}\: \\ $$$${its}\:{center}\:{are}\:{O}\begin{pmatrix}{{r}}\\{{r}}\end{pmatrix}\:\:{and}\:{its}\:{equation} \\ $$$${will}\:{be}\:\left({x}−{r}\right)^{\mathrm{2}} +\left({y}−{r}\right)^{\mathrm{2}} ={r}^{\mathrm{2}.} \\ $$$${by}\:{replacing}\:\:{y}=\frac{\mathrm{1}}{{x}}\:{in}\:{this}\:{equation} \\ $$$${we}\:{have}\:\:\:\left({x}−{r}\right)^{\mathrm{2}} +\left(\frac{\mathrm{1}}{{x}}−{r}\right)^{\mathrm{2}} ={r}^{\mathrm{2}} \\ $$$${x}^{\mathrm{2}} \left({x}−{r}\right)^{\mathrm{2}} +\left(\mathrm{1}−{rx}\right)^{\mathrm{2}} ={r}^{\mathrm{2}} {x}^{\mathrm{2}} \\ $$$${x}^{\mathrm{4}} −\mathrm{2}{rx}^{\mathrm{3}} +{r}^{\mathrm{2}} {x}^{\mathrm{2}} −\mathrm{2}{rx}+\mathrm{1}=\mathrm{0} \\ $$$${or} \\ $$$${x}^{\mathrm{2}} {r}^{\mathrm{2}} −\mathrm{2}{x}\left({x}^{\mathrm{2}} +\mathrm{1}\right){r}+\left(\mathrm{1}+{x}^{\mathrm{4}} \right)=\mathrm{0} \\ $$$${r}=\frac{−{b}\pm\sqrt{{b}^{\mathrm{2}} −\mathrm{4}{ac}}}{\mathrm{2}{a}}=\frac{−\mathrm{2}{x}\left({x}^{\mathrm{2}} +\mathrm{1}\right)\pm\sqrt{\mathrm{4}{x}^{\mathrm{2}} \left({x}^{\mathrm{2}} +\mathrm{1}\right)^{\mathrm{2}} −\mathrm{4}{x}^{\mathrm{2}} \left(\mathrm{1}+{x}^{\mathrm{4}} \right)}}{\mathrm{2}{x}^{\mathrm{2}} } \\ $$$${r}={x}+\frac{\mathrm{1}}{{x}}\pm\sqrt{\mathrm{2}} \\ $$$${p}=\mathrm{2}\pi{r}=\mathrm{2}\pi\left({x}+\frac{\mathrm{1}}{{x}}\pm\sqrt{\mathrm{2}}\right) \\ $$$$\frac{{dp}}{{dx}}=\mathrm{0}\: \\ $$$$\mathrm{2}\pi\left(\mathrm{1}−\frac{\mathrm{1}}{{x}^{\mathrm{2}} }\right)=\mathrm{0} \\ $$$${x}^{\mathrm{2}} −\mathrm{1}=\mathrm{0}\:\:\:\:\:\:\:{x}=\mathrm{1}\:\:\:\:{x}=−\mathrm{1} \\ $$$${r}=\mathrm{1}+\frac{\mathrm{1}}{\mathrm{1}}\pm\sqrt{\mathrm{2}=}\mathrm{2}\pm\sqrt{\mathrm{2}}\:\:\:\:\blacksquare \\ $$

Commented by mrW1 last updated on 04/Feb/17

you are basically right. I have cut  short the calculation, because we  know due to symmetry that the  circles tangent the curve at (1,1)  as well as at (−1,−1).

$${you}\:{are}\:{basically}\:{right}.\:{I}\:{have}\:{cut} \\ $$$${short}\:{the}\:{calculation},\:{because}\:{we} \\ $$$${know}\:{due}\:{to}\:{symmetry}\:{that}\:{the} \\ $$$${circles}\:{tangent}\:{the}\:{curve}\:{at}\:\left(\mathrm{1},\mathrm{1}\right) \\ $$$${as}\:{well}\:{as}\:{at}\:\left(−\mathrm{1},−\mathrm{1}\right). \\ $$

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