x=Ax0
y=arc sin [b/((b^2 +c^2)^(1/2))]
sin y = b/((b^2 +c^2)^(1/2))
cos y = c/((b^2 +c^2)^(1/2))
a=e^(-x) *(b sin x +c cos x)
=(b^2 +c^2)^(1/2)* e^(-x) *(sin y sin x + cos y cos x)
=(b^2 +c^2)^(1/2)* e^(-x) *cos(x+y)=a
e^(-x) *cos(x+y)=a /[(b^2 +c^2)^(1/2)]
small x,x=-y+arc cos {a /[(b^2 +c^2)^(1/2)]}
large x, a small, x+y=3.1416/2, x=3.1416/2 - y