Practicing Success
The lengths of tangent, subtangent, normal and subnormal for the curve $y=x^2+x-1$ at (1, 1) are A, B, C and D respectively, then their increasing order is |
B, D, A, C B, A, C, D A, B, C, D B, A, D, C |
B, A, D, C |
The equation of the curve is $y=x^2+x-1$ ∴ $\frac{d y}{d x}=2 x+1 \Rightarrow\left(\frac{d y}{d x}\right)_{(1,1)}=3$ Now, A = Length of the tangent at (1, 1) $\Rightarrow A=\frac{\sqrt{1+\left(\frac{d y}{d x}\right)^2}}{\frac{d y}{d x}}=\frac{\sqrt{1+3^2}}{3}=\frac{\sqrt{10}}{3}$ B = Length of the subtangent at (1, 1) $\Rightarrow B=\frac{y}{\frac{d x}{d y}}=\frac{1}{3}$ C = Length of the normal at (1, 1) $\Rightarrow C =\sqrt{1+\left(\frac{d y}{d x}\right)^2}=\sqrt{1+3^2}=\sqrt{10}$ D = Length of the subnormal at (1, 1) $\Rightarrow D =y \frac{d y}{d x}=1 \times 3=3$ Thus, we have B < A < D < C. |