- Thread starter Seiko
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The diagram implies a + b = $60^\circ\!$.

If the lower left to upper right line segment is a diameter of the circle, a = $25^\circ\!$ and b = $35^\circ\!$.

The answer says a=35

The diagram implies a + b = $60^\circ\!$.

If the lower left to upper right line segment is a diameter of the circle, a = $25^\circ\!$ and b = $35^\circ\!$.

b=25

But I don’t know why

I think there is something that was not stated in that question, but AB should have been stated to be the diameter, do you have explanation for that case ? (Nothing stated to be the diameter)As stated byskipjack, if chord AB is a diameter, then $\angle{ACB} = 90^\circ \implies \angle{BAC} = 25^\circ$

The incribed angle marked $a$ and $\angle{BAC}$ intercept the same arc $\implies a = 25^\circ$. From there you can determine angle $b$ ...

View attachment 11101

Thanks in advance

Thanks a millionWithout the diameter being given, the most that can be shown is that $a + b = 60^\circ$ (as the angles of a triangle add to give $180^\circ$).