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Question Number 43180 by gunawan last updated on 08/Sep/18

The smallest and the largest values of  tan^(−1) (((1−x)/(1+x))) ,  0≤ x ≤ 1  are

Thesmallestandthelargestvaluesoftan1(1x1+x),0x1are

Commented by maxmathsup by imad last updated on 08/Sep/18

let ϕ(x)=arctan(((1−x)/(1+x)))   with x∈[0,1] changement x =cosθ give  ϕ(x)=ϕ(cosθ) =arctan(((1−cosθ))/(1+cosθ))) =arctan( tan^2 ((θ/2)))  we have 0≤x≤1 ⇒0≤θ≤(π/2) ⇒ 0≤tan((π/2))≤1 ⇒0≤tan^2 ((θ/2))≤1 ⇒  0≤ϕ(cosθ)≤(π/4) ⇒ ϕ(x)∈[0,(π/4)](both function tan and arctan are increasing)

letφ(x)=arctan(1x1+x)withx[0,1]changementx=cosθgiveφ(x)=φ(cosθ)=arctan(1cosθ)1+cosθ)=arctan(tan2(θ2))wehave0x10θπ20tan(π2)10tan2(θ2)10φ(cosθ)π4φ(x)[0,π4](bothfunctiontanandarctanareincreasing)

Answered by tanmay.chaudhury50@gmail.com last updated on 08/Sep/18

y=tan^(−1) 1−tan^(−1) x  y=(Π/4)−tan^(−1) x  when x=0  tan^(−1) x=0  y=(Π/4)  when x=1     tan^(−1) (1)=(Π/4)  so y=(Π/4)−(Π/4)=0  so max of y=(Π/4)    min of y=0

y=tan11tan1xy=Π4tan1xwhenx=0tan1x=0y=Π4whenx=1tan1(1)=Π4soy=Π4Π4=0somaxofy=Π4minofy=0

Commented by gunawan last updated on 08/Sep/18

Nice Sir

NiceSir

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