Q37 / 60 · JEE Advanced 2015 (Paper 1) · Mathematics

Let fx=sin⁡π6sin⁡π2sin⁡x, for all x∈R and gx=π2sin⁡x, for all x∈R. Let fog x denote fgx and gof x denote gfx. Then which of the following is (are) true?

  • Correct answer
    A
    Range of f  is -12,12
  • Correct answer
    B
    Range of fog  is -12,12
  • Correct answer
    C
    limx→0⁡f(x)g(x)=π6
  • D
    There is an x∈R  such that g o fx=1

Solution

fx=sin⁡π6sin⁡π2sin⁡x
-π2≤π2sin⁡x≤π2
-1≤sin⁡π2sin⁡x≤1
sin⁡-π6≤sin⁡π6sin⁡π2sin⁡x≤sin⁡π6
-12≤sin⁡π6sin⁡π2sin⁡x≤12
fgx=sin⁡π6sin⁡π2sin⁡π2sin⁡x
-1≤sin⁡π2sin⁡x≤1
-π2≤π2sin⁡π2sin⁡x≤π2
-1≤sin⁡π2sin⁡π2sin⁡x≤1
-π6≤π6sin⁡π2sin⁡π2sin⁡x≤π6
-12≤sin⁡π6sin⁡π2sin⁡π2sin⁡x≤12
limx→0fxgx=limx→0⁡sin⁡π6sinπ2sinxπ2sin⁡x×π6sin⁡π2sin⁡xπ6sin⁡π2sin⁡x

=limx→0⁡π6sin⁡π2sin⁡xπ2sin⁡x=π6.

Figure
Range of gfx is π2sin⁡-12,π2sin⁡12
=-π2sin⁡12,π2sin⁡12
Hence, 1, does not belong to this range.

Hence, the correct answer is options: 'A', 'B', 'C'.

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