Có: ^xOy+^yOz=180
<=> ^xOm+yOm+^yOn+^nOz=180
<=> 2^mOy+2^yOn=180
<=> 2(^mOy+^yOn)=180
<=>mOn=90
Ta có; xOy + yOz = 180o (kề bù)
=>xOm + yOm + yOn + nOz = 180o
Mà xOm = mOy (gt)
yOn = nOz(gt)
Nên 2yOm + 2yOn = 180o
=> 2(yOm + yOn) = 180o
=> yOm + yOn = 90o
=> mOn = 90o (đpcm)