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Problem
Problem
If a
△
A
B
C
\triangle ABC
△
A
B
C
, inscribed in a fixed circle, is slightly varied in such a way as to have its vertices always on the circle, show that
d
a
cos
A
+
d
b
cos
B
+
d
c
cos
C
=
0
\frac{da}{\cos A} + \frac{db}{\cos B} + \frac{dc}{\cos C} = 0
c
o
s
A
d
a
+
c
o
s
B
d
b
+
c
o
s
C
d
c
=
0
.
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