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Let $$f: \mathbb{R} \rightarrow \mathbb{R}$$ be a function defined as $$f(x)=a \sin \left(\frac{\pi[x]}{2}\right)+[2-x], a \in \mathbb{R}$$ where $$[t]$$ is the greatest integer less than or equal to $$t$$. If $$\mathop {\lim }\limits_{x \to -1 } f(x)$$ exists, then the value of $$\int\limits_{0}^{4} f(x) d x$$ is equal to

JEE · Math · previous-year question

  1. A.$$-$$1
  2. B.$$-$$2correct
  3. C.1
  4. D.2

Answer

B. $$-$$2

Explanation

$$f(x) = a\sin \left( {{{\pi [x]} \over 2}} \right) + [2 - x]\,a \in R$$ Now, $$\because$$ $$\mathop {\lim }\limits_{x \to - 1} f(x)$$ exist $$\therefore$$ $$\mathop {\lim }\limits_{x \to - {1^ - }} f(x) = \mathop {\lim }\limits_{x \to - {1^ + }} f(x)$$ $$ \Rightarrow a\sin \left( {{{ - 2\pi } \over 2}} \right) + 3 = a\sin \left( {{{ - \pi } \over 2}} \right) + 2$$ $$ \Rightarrow - a = 1 \Rightarrow a = - 1$$ Now, $$\int_0^4 {f(x)dx = \int_0^4 {\left( { - \sin \left( {{{\pi [x]} \over 2}} \right) + [2 - x]} \right)dx} } $$ $$ = \int_0^1 {1dx + \int_1^2 { - 1dx + \int_2^3 { - 1dx + \int_3^4 {(1 - 2)dx} } } } $$ $$ = 1 - 1 - 1 - 1 = - 2$$

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