By letting

we get derivatives


a) Substitute these into the differential equation. After a lot of simplification, the equation reduces to

Examine the lowest degree term
, which gives rise to the indicial equation,

with roots at r = 0 and r = 4/5.
b) The recurrence for the coefficients
is

so that with r = 4/5, the coefficients are governed by

c) Starting with
, we find


so that the first three terms of the solution are

Answer:
Step-by-step explanation:
sin(90-a)=cos
cos = angle adjacent
theta=BC/CA
Remark
I would have had the answer a whole lot sooner if I would have read the question properly. The figure in the circle is called a cyclic quadrilateral. It has the odd property that the angles that are opposite each other add up to 180o.
So DEB + DCB = 180o
DEB = 180 - 87
DEB = 93o
Note: The arcs marked 60 and 76 have nothing whatever to do with this problem.
What does it constraints?