Let the congruent lengths be 1 unit.
(1/sin66) = (BC/sin84)
BC = 1.08863974644 units.
(AC)² = 1.08863974644²+1-2*1.08863974644cos78
AC = 1.31622742593 units.
(1.31622742593/sin78) = (1/sina)
a = 48°
a is angle ACB.
Therefore, the required angle, angle ACD is;
a-30
= 48-30
= 18°
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