§14]
GREAT AND SMALL CIRCLES.
7
. Then
,
,
,
are all perpendicular to
, because
the planes
and
are perpendicular to
; therefore
is parallel to
, and
is parallel to
. Therefore the angle
the angle
(Euclid, XI. 10). Hence,
, (Plane Trigonometry, Art. 18);
therefore,
.