An equilateral triangle ABC shares a common side BC with a square BCDE, as pictured. What is the number of degrees in angle DAE (not pictured)?
For convenience sake...let the side of the square = 1
Then the height of the equilateral triangle = √3/2
Position D at (0,0)
Then A will have the coordinates ( 1/2, 1 + √3/2)
And we can find 1/2 of the measure of angle DAE as
arctan [ (1/2) / ( 1 +√3/2 ) ] = 15°
So...DAE will be twice this = 30°
Actually....this could be done more simply
Draw AD....now....we have ΔACD with angle ACD = 150°
And since CD = CA, then angle CAD and And ADC are equal
So angle CAD = [180 -150 ] / 2 = 30/2 = 15°
And if we draw, AE, by the same process, in Δ ABE, angle EAB = 15°
So CAB = 60° = angle CAD + angle DAE + angle EAB
60 = 15 + angle DAE + 15
60 = 30 + angle DAE
30° = angle DAE