You have asked the exact question that I usually expect a student to ask - I always say - when something is not given, you cannot assume it based on the diagram... I will put forth my observation - you can accept if you like Since this is probably a forwarded puzzle image (and not school exam), we can say the dot at 'O' denotes the larger circle's centre. If you accept that, then, it is clear - the point of contact of the circles at the far left end is touching tangentially, and the horizontal line is the diameter of the larger circle, so the smaller circle center is also along the horizontal line (also denoted by a dot at O' in the original diagram). Secondly, since the vertical line is a chord for the smaller circle, and passing through the centre O, and is equidistant from the larger circle (both are 10 cm) - it has to be perpendicular (90 degree) to the horizontal line... Keep smiling HR
