If two chords AB and CD of a circle AYDZBWCX intersect at right angles (see Fig.10.18), prove that arc CXA + arc DZB = arc AYD + arc BWC = semicircle.
Given: In circle AYDZBWCX two chords AB and CD intersect at right angles
To Prove: Arc CXA + Arc DZB = arc AYD + arc BWC = Semicircle
Proof:
Draw Diameter EF || CD having centre M
Since CD || EF
Arc EC = DF … (i)
Arc ECXA = arc EWB (Symmetrical about diameter of circle)
Arc AF = arc FB … (ii) (Symmetrical about diameter of circle)
We know that arc ECXAYDF = Semicircle
Arc EA + arc AF = Semicircle
arc EC + arc CXA + arc FB = semicircle (from ii, Arc AF = arc FB)
arc DF + arc CXA + arc FB = semicircle (from i, Arc EC = arc DF)
=> arc DF + arc FB + arc CXA = semicircle
Arc DZB + arc CXA = semicircle. (First statement is proved)
Now, we know that a circle is divided into two semi-circles.
Therefore the remaining portion of the circle (other than, Arc DZB + arc CXA) is also equal to a semi-circle
So, Arc AYD + arc BWC = semicircle
So we get: Arc CXA + Arc DZB = arc AYD + arc BWC = Semicircle
Hence proved