Practicing Success
For the curve $y=c e^{x / a}$, which one of the following is incorrect? |
subtangent is constant subnormal varies as the square of the ordinate tangent at $\left(x_1, y_1\right)$ on the curve intersects the x-axis at a distance of $\left(x_1-a\right)$ from the origin equation of normal at the point where the curve cuts y-axis is $c y+a x=c$ |
equation of normal at the point where the curve cuts y-axis is $c y+a x=c$ |
We have, $y=c e^{x / a} \Rightarrow \frac{d y}{d x}=\frac{c}{a} e^{x / a} \Rightarrow \frac{d y}{d x}=\frac{1}{a} y$ ∴ $\frac{y}{d y / d x}$ = a = Const. ⇒ Subagent = Const. Length of the subnormal $=y \frac{d y}{d x}=y \times \frac{y}{a}=\frac{y^2}{a} \propto$ (Square of the ordinate) Equation of the tangent at $\left(x_1, y_1\right)$ is $y-y_1=\frac{y_1}{a}\left(x-x_1\right)$ This meets x-axis at a point given by $-y=\frac{y_1}{a}\left(x-x_1\right) \Rightarrow x=x_1-a$ The curve meets y-axis at (0, c) ∴ $\left(\frac{d y}{d x}\right)_{(0, c)}=c / a$ So, equation of the normal at (0, c) is $y-c=-\frac{1}{c / a}(x-0) \Rightarrow a x+c y=c^2$ |