12th Class Mathematics Sample Paper Mathematics Sample Paper-15

  • question_answer
    Find the equation of the curve passing through origin if the slope of the tangent to the curve at any point (x, y) is equal to the square of the difference of the abscissa and ordinate of the point.

    Answer:

    Slope of tangent to the curve is \[\frac{dy}{dx}\]and the square of the difference of the abscissa and the ordinate is\[{{(x-y)}^{2}}\]. According to the question, slope of the tangent to the curve is equal to the square of the difference of the abscissa and ordinate of the point. \[\therefore \]                  \[\frac{dy}{dx}={{(x-y)}^{2}}\]                      ?(i) Put                   \[z=x-y\] \[\therefore \]      \[\frac{dz}{dx}=1-\frac{dy}{dx}\] [on differentiating w.r.t. x] \[\Rightarrow \]   \[\frac{dy}{dx}=1-\frac{dz}{dx}\] Now, substituting\[\frac{dy}{dx}=1-\frac{dz}{dx}\]and \[z=x-y\]in Eq. (i), we get \[1-\frac{dz}{dx}={{z}^{2}}\Rightarrow \frac{dz}{dx}=1-{{z}^{2}}\] By separating the variables, we get \[\frac{dz}{1-{{z}^{2}}}=dx\] On integrating both sides, we get \[\int{\frac{dz}{1-{{z}^{2}}}=\int{dx\Rightarrow \frac{1}{2}\log }\left| \frac{1+z}{1-z} \right|\,\,=x+{{C}_{1}}}\] \[\left[ \because \int{\frac{dx}{{{a}^{2}}-{{x}^{2}}}=\frac{1}{2a}\log \,\,\left| \frac{a+x}{a-x} \right|+C} \right]\] \[\Rightarrow \]   \[\log \,\,\left| \frac{1+z}{1-z} \right|\,\,=2\,(x+{{C}_{1}})\] Now, substituting, \[z=x-y,\] we get \[{{\log }_{e}}\left| \frac{1+(x-y)}{1-(x-y)} \right|\,\,=2x+{{C}_{2}}\] \[[\because {{C}_{2}}=2{{C}_{1}}]\]             \[\Rightarrow \]               \[\frac{1+(x-y)}{1-(x-y)}=\pm \,{{e}^{2x\,\,+\,\,{{C}^{2}}}}\] \[=\pm \,\,C{{e}^{2x}}\]                      \[[\text{here},\,\,C={{e}^{{{C}_{2}}}}]\] It is also given that curve passing through origin.             \[\therefore \]\[\frac{1+(0-0)}{1-(0-0)}=\pm \,\,C{{e}^{0}}\Rightarrow C=\pm \,\,1\]                 \[[\because {{e}^{0}}=1]\]             \[\therefore \]                  \[\frac{1+(x-y)}{1-(x-y)}=\pm \,\,{{e}^{2x}}\]             \[\Rightarrow \]   \[\pm \,\,{{e}^{2x}}(1-x+y)-1=x-y\] \[\Rightarrow \]   \[(1+x-y)=\pm \,\,(1-x+y){{e}^{2x}}\]


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