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JEE Advanced · Mathematics · 8. Trigonometric Equations

Let fx=sinπcosx and gx=cos2πsinx be two functions defined for x>0. Define the following sets whose elements are written in the increasing order:
X=x:fx=0, Y=x:f'x=0
Z=x:gx=0, W=x:g'x=0
List- I contains the sets X, Y, Z and W. List- II contains some information regarding these sets.
 
List- I List- II    
I X P π2,3π2, 4π, 7π
II Y Q an arithmetic progression
III Z R NOT an arithmetic progression
IV W S π6,7π6,13π6
    T π3,2π3, π
    U π6,3π4

Which of the following is the only correct combination?

  1. A IV-Q, T
  2. B IV-P, R, S
  3. C III-R, U
  4. D III-P, Q, U
Verified Solution

Answer & Solution

Correct Answer

(B) IV-P, R, S

Step-by-step Solution

Detailed explanation

I fx=sinπcosx  and X=x:fx=0
fx=0
sinπcosx=0
πcosx=nπ
cosx=n
cosx=0, 1, -1
x=nπ2
X=nπ2, nNX=π2, π,3π2, 2π
I-P, Q
II gx=cos2πsinx and Z=x:gx=0
cos2πsinx=0
2πsinx=2n+1π2
sinx=2n+14
sinx=-14,14,-34,34
Z=nπ±sin-114, nπ±sin-134
II-Q, T
III fx=sinπcosx and Y=x:f'x=0
f'x=cosπcosx.-πsinx=0
now cosπcosx=0πcosx=2n+1π2
cosx=2n+12
cosx=-12,12x=nπ±π3
or sinx=0x=nπ
hence Y=nπ, nπ±π3
Y=π3,2π3, π,4π3,5π3, 2π
IIIR
IV gx=cos2πsinx and W=x:g'x=0
g'x=-sin2πsinx2πcosx=0
now cosx=0x=2n+1π2
or sin2πsinx=0
2πsinx=nπ
sinx=n2
sinx=-1, -12, 0,12, 1
x=nπ2, nπ±π6
W=nπ2, nπ±π6
W=π6,π2,5π6,π, 7π6,3π2.
IV-P, R, S
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