JEE Challenger
More from Application of Derivatives

Intersection of Tangent to Exponential Curve with Secant Line

The tangent to the curve y=exy = e^x drawn at the point (c,ec)(c, e^c) intersects the line joining the points (c−1,ec−1)(c-1, e^{c-1}) and (c+1,ec+1)(c+1, e^{c+1})

Options

A

on the left of x=cx = c

Correct
B

on the right of x=cx = c

C

at no point

D

at all points

Step-by-Step Solution

To find the point of intersection between the tangent line and the secant line, we first determine the equations of both lines.

  1. Equation of the Tangent Line:
    The given curve is y=exy = e^x. The derivative with respect to xx is:
    dydx=ex\frac{dy}{dx} = e^x
    At the point (c,ec)(c, e^c), the slope of the tangent is:
    m1=dydx∣x=c=ecm_1 = \left.\frac{dy}{dx}\right|_{x=c} = e^c
    Using the point-slope form, the equation of the tangent line is:
    y−ec=ec(x−c)  ⟹  y=ec(x−c+1)y - e^c = e^c(x - c) \implies y = e^c(x - c + 1)

  2. Equation of the Secant Line:
    The line joins the points (c−1,ec−1)(c-1, e^{c-1}) and (c+1,ec+1)(c+1, e^{c+1}). The slope of this secant line is:
    m2=ec+1−ec−1(c+1)−(c−1)=ec(e−e−1)2m_2 = \frac{e^{c+1} - e^{c-1}}{(c+1) - (c-1)} = \frac{e^c(e - e^{-1})}{2}
    Using the point (c−1,ec−1)(c-1, e^{c-1}), the equation of the secant line is:
    y−ec−1=m2(x−(c−1))y - e^{c-1} = m_2(x - (c-1))
    y=ec(e−e−1)2(x−c+1)+ec−1y = \frac{e^c(e - e^{-1})}{2}(x - c + 1) + e^{c-1}

  3. Finding the Point of Intersection:
    Equating the two expressions for yy:
    ec(x−c+1)=ec(e−e−1)2(x−c+1)+ec−1e^c(x - c + 1) = \frac{e^c(e - e^{-1})}{2}(x - c + 1) + e^{c-1}
    Dividing the entire equation by ece^c:
    (x−c+1)=(e−e−12)(x−c+1)+e−1(x - c + 1) = \left(\frac{e - e^{-1}}{2}\right)(x - c + 1) + e^{-1}
    Let u=x−cu = x - c. Then:
    (u+1)(1−e−e−12)=e−1(u + 1)\left(1 - \frac{e - e^{-1}}{2}\right) = e^{-1}
    (u+1)(2−e+e−12)=1e(u + 1)\left(\frac{2 - e + e^{-1}}{2}\right) = \frac{1}{e}
    Solving for u+1u + 1:
    u+1=2e(2−e+e−1)=22e−e2+1u + 1 = \frac{2}{e(2 - e + e^{-1})} = \frac{2}{2e - e^2 + 1}

Now, let us evaluate the denominator:
2e−e2+1=1−(e−1)2+1=2−(e−1)22e - e^2 + 1 = 1 - (e - 1)^2 + 1 = 2 - (e - 1)^2
Since e≈2.718e \approx 2.718, we have:
(e−1)2≈(1.718)2≈2.952>2(e - 1)^2 \approx (1.718)^2 \approx 2.952 > 2
Therefore:
2e−e2+1<02e - e^2 + 1 < 0
Since the numerator is positive (2>02 > 0) and the denominator is negative, we have:
u+1<0  ⟹  u<−1u + 1 < 0 \implies u < -1
Substituting back u=x−cu = x - c:
x−c<−1  ⟹  x<c−1<cx - c < -1 \implies x < c - 1 < c

Hence, the xx-coordinate of the point of intersection satisfies x<cx < c, which means the intersection lies on the left of x=cx = c.

Therefore, the correct option is (A).