Question 1068288
A and B are points on both circles.
C is on one circle,
where BC is a diameter.
Since BC is a diameter, angle BAC is an inscribed angle
intercepting an arc that is half of that circle.
The measure of BAC is half of the intercepted arc,
or {{{180^o/2=90^o}}} .
The same reasoning with diameter BD on the other circle
proves that angle BAD measures {{{90^o}}} .
Since BAC and BAD both measure {{{90^o}}} ,
their sum, angle CAD measures
{{{90^o+90^o=180^o}}} ,
meaning that rays AC and AD are two "halves" of the same line CD.