MCQ
Let $f : R \times R$ be a function defined by $ \text{f}(\text{x}) = \cos(2\text{x} + 5)$, then $f$ is:
  • A
    injective
  • B
    surjective
  • C
    bijective
  • None of these

Answer

Correct option: D.
None of these
Given, $ \text{f}(\text{x}) = \cos(2\text{x} + 5)$
Period of $\text{f}(\text{x})=\frac{2\pi}{5}$
Since $f(x)$ is a periodic function with period $\text{f}(\text{x})=\frac{2\pi}{5}$
so it is not injective.
The function $f$ is not surjective also as its range $[-1, 1]$ is a proper subset of its co$-$domain $R.$

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