At what points on the following curves, is the tangent parallel to the x–axis?
y = 12(x + 1) (x – 2) on [–1, 2]
First, let us write the conditions for the applicability of Rolle’s theorem:
For a Real valued function ‘f’:
a) The function ‘f’ needs to be continuous in the closed interval [a,b].
b) The function ‘f’ needs differentiable on the open interval (a,b).
c) f(a) = f(b)
Then there exists at least one c in the open interval (a,b) such that f’(c) = 0.
Given function is:
⇒ y = 12(x + 1)(x – 2) on [ – 1,2]
We know that polynomials are continuous and differentiable over R.
Let’s check the values of y at the extremums
⇒ y( – 1) = 12( – 1 + 1)( – 1 – 2)
⇒ y( – 1) = 12(0)( – 3)
⇒ y( – 1) = 0
⇒ y(2) = 12(2 + 1)(2 – 2)
⇒ y(2) = 12(3)(0)
⇒ y(2) = 0
We got y( – 1) = y(2). So, there exists a c such that f’(c) = 0.
For a curve g to have a tangent parallel to the x – axis at point r, the criteria to be satisfied is g’(r) = 0.
⇒ y’(x) = 0
⇒
⇒
⇒ ((x + 1)×1) + ((x – 2)×1) = 0
⇒ x + 1 + x – 2 = 0
⇒ 2x – 1 = 0
⇒ 2x = 1
⇒
The value of y is
⇒
⇒
⇒ y = – 27
The point at which the curve has tangent parallel to x – axis is .