Discussion Forum : Application Of Derivatives
Question -


Let f (x) = sinx + ax + b. Then f(x) = 0 has

Options:
A .   only  one  real root which is  positive if a > 1, b < 0
B .   only  one  real root which is negative if a > 1, b < 0
C .   only one real root which is negative if a < 1, b > 0
D .   CAN'T SAY ANYTHING
Answer: Option A
:
A

f'(x) = - cosx + a, if a > 1,then f(x) entirely increasing. So f(x) =0 has only one real root, which is positive if f(0) < 0 and negative if f(0) > 0.


Similarly when a < -1. Then f(x) entirely decreasing. So f(x) has only one real root which is negative if f(0) < 0 and positive if f(0) > 0



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