Let y=y(x) be the solution of the differential equation dydx=1+xey−x,−√2<x<√2,y(0)=0, then the minimum value of y(x),x∈(−√2,√2) is equal to
A
(2+√3)+loge2
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B
(1+√3)−loge(√3−1)
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C
(1−√3)−loge(√3−1)
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D
(2−√3)−loge2
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Solution
The correct option is C(1−√3)−loge(√3−1) dydx=1+xey−x ⇒e−ydydx=e−y+xe−x
Let e−y=t⇒−e−ydydx=dtdx⇒−dtdx=t+xe−x ⇒dtdx+t=−xe−x ∴ Integrating function e∫1dx=ex ∴ Solution is : t⋅ex=∫−xe−x.exdx ⇒t⋅ex=−x22+c ⇒ex−y=−x22+c ∵y(0)=0⇒c=1 ∴ex−y=1−x22⇒y(x)=x−log(1−x22)
Now y′(x)=1−11−x22(0−x) =2−x2+2x2−x2 =(x−1)2−3x2−2 ∴y′(x)=0⇒x=1±√3
∵x∈(−√2,√2)∴x=1−√3 is point of local minima ∴ Minimum value of y(x)=y(1−√3) =(1−√3)−loge(1−(1−√3)22) =(1−√3)−loge(√3−1)