Verify Lagrange’s mean value theorem for the following functions on the indicated intervals. In each find a point ‘c’ in the indicated interval as stated by the Lagrange’s mean value theorem :
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.
Here,
⇒ 25 – x2 >0
⇒ x2 < 25
⇒ – 5 < x < 5
∴ f(x) is continuous in [ – 3, 4]
Differentiating with respect to x:
Here also,
⇒ – 5 < x < 5
∴ f(x) is differentiable in ( – 3, 4)
So both the necessary conditions of Lagrange’s mean value theorem is satisfied.
On differentiating with respect to x:
For f’(c), put the value of x=c in f’(x):
For f(4), put the value of x=4 in f(x):
⇒ f(4) = 3
For f( – 3), put the value of x= – 3 in f(x):
⇒ f( – 3) = 4
Squaring both sides:
⇒ 49c2 = 25 – c2
⇒ 50c2 = 25
Hence, Lagrange’s mean value theorem is verified.