NCERT Solutions
Class 12 Maths
Application of Derivatives

Ex.Misc.Q.13
Find the points at which the function f given by f(x) = (x - 2)4(x + 1)3 has
(i) local maxima
(ii) local minima
(iii) point of inflexion
The given function is f(x) = (x - 2)4(x + 1)3
Now, f’(x) = 4(x - 2)3(x + 1)3 + 3(x - 2)4(x + 1)2
= (x - 2)3(x + 1)2[4(x + 1) + 3(x – 2)]
= (x - 2)3(x + 1)2(7x - 2)3
Now, f’(x) = 0
⟹ (x - 2)3(x + 1)2(7x - 2)3 = 0
⟹ x = -1, , 2
Now, for values of x close to and to the left of
, f’(x) > 0
Also, for values of x close to and to the right of
, f’(x) < 0
Thus, x = is the point of local maxima.
Now, for values of x close to 2 and to the left of 2, f’(x) < 0
Also, for values of x close to 2 and to the right of 2, f’(x) > 0
Thus, x = 2 is the point of local minima.
Now, as the value of x varies through −1, f’(x) does not changes its sign.
Thus, x = −1 is the point of inflexion.