Proof `:` Considering `/_ABC ` and the arc AXB intercepted by it, we see from the figures that arc AXB may be a semicircle, may be a minor arc or may be a major arc. We deal with these cases separately .
(a) When `/_ ABC` intercepts a semicircle , [as in figure (a) ] the chord BA passes through the centre.
In that case, `/_ABC = 90^(@)` ....(Tangent theorem )
Also since m ( arc BXA `= 180^(@)`
`:. (1)/(2) m ` (arc BXA ) ` = (1)/(2) xx180^(@) = 90^(@)`
`:.` m ` /_ABC = (1)/(2) m ` (arc BXA )
(b) When `/_ ABC ` intercepts a minor arc, [ as in figure (b) ] the centre lies in the exterior of `/_ ABC `.
We have `, m /_ABC = 90^(@) - m /_ABO`
`:. m /_ ABO = 90^(@) - m /_ ABC `
`:. m /_BAO = 90^(@) - m /_ ABC ` ....(`Delta ABC ` is isosceles )
`m /_ABO + m /_BAO = 180^(@) - 2m/_ABC `
`:. 180^(@) - m /_ BOA = 180^(@) - 2m /_ABC `
`:. 180^(@) - m /_BOA = 180^(@) - 2m/_ABC `
`:. m /_BOA = 2m/_ABC `
But `m/_BOA = m (arc AXB ) `
`:. m/_ABC = (1)/(2) m ` (arc AXB)
(c) When `/_ABC ` intercepts a major arc [as in figure (c ) the centre lies in the interior of `/_ABC ` .
We then have `/_DBA ` intercepting a minor arc BYA .
`:. m/_DBA = (1)/(2) ` m (arc BYA )
`:. 180^(@) - m/_ABC = (1)/(2) [ 360^(@) - m ` (arc AXB ) ]
`= 180^(@) - (1)/(2)` m (arc AXB )
`:. m /_ABC = (1)/(2) ` m (arc AXB )
Thus, in every case we have shown that `m/_ABC = (1)/(2)` m (arc AXB ) .