AC1 + AC2 = 3 ... obviously, right?
O1A and O2A are parallel, being radii at the point the two circles touch.
Angles AO1C1 and AO2C2 are equal, being opposite angles. Let's call this angle
because the first angle in any maths problem is always called
.
Angle AD1C1 = half angle AO1C1, which is a thing you are supposed to learn.
And now it's just trig ... I think