ok cool. thanks for the answer. but then I want to derive it and find the critical point.
tanθ=5.6X/(X^2-d^2-5.6d)
sec^2 θ dθ/dX=5.6((-d^2-5.6d-X^2)/(X^2-d^2-5.6d)^2 )
-d^2-5.6d-X^2=0
X^2=-d^2-5.6d
X=√(-d^2-5.6d)