Find points on the curve y = x3 – 3x, where the tangent to the curve is parallel to the chord joining (1, – 2) and (2, 2).
Lagrange’s mean value theorem states that if a function f(x) is continuous on a closed interval [a, b] and differentiable on the open interval (a, b), then there is at least one point x=c on this interval, such that
f(b)−f(a)=f′(c)(b−a)
This theorem is also known as First Mean Value Theorem.
Let f(x) = x3 – 3x on [1, 2]
This interval [a, b] is obtained by x – coordinates of the points of the chord.
Every polynomial function is continuous everywhere on (−∞, ∞) and differentiable for all arguments.
Here, f(x) is a polynomial function. So it is continuous in [1, 2] and differentiable in (1, 2). So both the necessary conditions of Lagrange’s mean value theorem is satisfied.
f(x) = x3 – 3x
Differentiating with respect to x:
f’(x) = 3x2 – 3
For f’(c), put the value of x=c in f’(x):
f’(c)= 3c2 – 3
For f(2), put the value of x=2 in f(x):
f(2)= (2)3 – 3(2)
= 8 – 6
= 2
For f(1), put the value of x=1 in f(x):
f(1) = (1)3 – 3(1)
= 1 – 3
= – 2
f’(c) = f(2) – f(1)
⇒ 3c2 – 3 = 2 – ( – 2)
⇒ 3c2 – 3 = 2 + 2
⇒ 3c2 = 4 + 3
We know that, the value of c obtained in Lagrange’s Mean value Theorem is nothing but the value of x – coordinate of the point of the contact of the tangent to the curve which is parallel to the chord joining the points (1, – 2) and (2, 2).
Now, Put this value of x in f(x) to obtain y:
y = x3 – 3x