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Let f:RR be a function such that f(x)=ax+3sin x+4 cos x.Then f(x) is invertible if

a
a∈(−5,5)
b
a∈(−∞,5)
c
a∈(−5,+∞)
d
none of these

detailed solution

Correct option is D

f′(x)=a+3cos ⁡x−4sin ⁡x        =a+5cos ⁡(x+α), where cos ⁡α=35For invertible, f′(x)≥0∀x or f′(x)≤0∀x i.e.,a+5cos⁡(x+α)≥0 or a+5cos⁡(x+α)≤0 i.e.,a≥−5cos⁡(x+α) or a≤−5cos⁡(x+α) i.e.,a≥5 or a≤−5 f (x) must be monotonic. Thus,

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