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Grade: 12


Consider a function g(x) which is defined and differentiable on (-8,8) and increasing in (1,2) and decreasing elsewhere. We construct another function f(x) = g(x) - (g(x))2 + (g(x))3. Find domain of f(x), it’s interval of monotonicity.

6 years ago

Answers : (1)

bharat bajaj
IIT Delhi
askIITians Faculty
122 Points
							f(x) = g(x) - (g(x))^2 + (g(x))^3
f'(x) = g'(x) ( 1 - 2 g(x) + 3g(x)^2)

Now, 3g(x)^2 - 2g(x) + 1 has Discriminant = 0. Hence, this means that this is either always positive or always negative.
g'(x) > 0 in the interval (1,2)
g'(x) < 0 in the interval (-8,1) U (2,8)
The domain of f(x) is same as that of g(x) which is (-8,8).
For the interval of monotonicity are :
We cannot clearly say that 3g(x)^2 - 2g(x) + 1 is positive or negative as we do not know much about g(x). Say it is positive. Hence,
f(x) is monotonically increasing in interval (1,2)
f(x) is monotically decreasing in interval (-8,1) U (2,8)
Bharat Bajaj
IIT Delhi
askiitians faculty
6 years ago
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