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)
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)