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Answer 1 · 2018-05-01T20:29:26

#DeltaADC# is an isosceles right triangle

#:.#

#angleADC = 45^@#

#angleACB = 60^@# because is it one of the 3 angles of an equilateral triangle.

#:.#

#angleDCB = 90^@+60^@ = 150^@#

Because #angleBDC# is opposite one of two equal sides of #Delta BDC#:

#angleBDC = (180^@-angleDCB)/2#

#angleBDC = (180^@-150^@)/2#

#angleBDC = 15^@#

#angle ADB = angle ADC - angleBDC#

#angle ADB = 45^@ - 15^@#

#angle ADB = 30^@# Q.E.D.

Answer 2 · 2018-05-01T20:39:41

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