m3ssi3r

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  1. m3ssi3r added a post in a topic   

    ok , here is my solution
    [set of 7 distinct real numbers S, x,y E S]
    To Prove : 0< (x-y)/(1+xy) < 1/sqrt(3)
    Proof:
    let there be 7 angles xi's i-> [1,7] // i.e, x1,x2,.....
    xi's are angles xi -> [-pi/2,pi/2] ie [-90 degree, 90 degree]

    let their corresponding tans inverse i.e tan(zi)'s be xi's // tan(z1)=x1,tan(z2)=x2,.....
    //here tan(zi) -> [-infinity,+infinity]

    We know the range of tan is -infinity to +infinity //hence constrain of distinct real numbers satisfied

    if we divide the interval of [-90 degrees , 90 degree] into 6 sub-intervals


    |--- |---|---|---|---|---|
    -90 -60 -30 0 30 60 90
    I II III IV V VI

    if we transform this

    we have 7 angles and six sub-intervals,
    from pigeon hole principle we can infer that two angles will be in same interval , let them be X and Y.
    we can see that difference between X and Y can't be greater than 30 degrees.

    let x=Tan(X) and y=Tan(Y)
    hence we have 0 < X-Y < pi/6 // pi = 180 degree
    //Since tan is monotonically increasing function we can apply tan to this inequality
    hence Tan(0) < Tan(X-Y) < Tan(pi/6)

    ==> 0 < Tan(X)-Tan(y)/(1+Tan(X)*Tan(Y) < 1/sqrt(3)

    ==> 0 < (x - y) /(1+xy) < 1/sqrt(3) //Since Tan(X)=x and Tan(Y) = y
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