Q54 / 91 · KVPY 2020 (SB/SX) · Mathematics

For θ∈[0,π], let f(θ)=sin(cosθ) and g(θ)=cos(sinθ). Let a=max0≤θ≤πfθ,b=min0≤θ=πfθ,c=max0≤9≤πgθ and d=min0<θ<πgθ. The correct
inequalities satisfied by a, b, c, d are

  • A
    b<d<c<a
  • B
    d<b<a<c
  • Correct answer
    C
    b<d<a<c
  • D
    b<a<d<c

Solution

f(θ)=sin(cosθ)

g(θ)=cos(sinθ)

f'(θ)=cos(cosθ)(-sinθ)<0∀θ∈[0,π]

∴f(θ) decreases monotonically

∴a=maxf(θ)=f(0)=sin1

b=minf(θ)=f(π)=-sin1

g'(θ)=-sin(sinθ)cosθ

Figure

g(θ)=1;g(π)=1;gπ2=cos1

∴c=maxg(θ)=1

d=ming(θ)=cos1

∴b<d<a<c

Hence, the correct answer is option: 'C'.

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