a, Do
$\widehat{aOb}$ kề bù vs $\widehat{cOb}$ nên
$\widehat{aOb}$ + $\widehat{cOb}$ = `180^{o}`
=> `70^{o}` + $\widehat{cOb}$ = `180^{o}`
=> $\widehat{cOb}$ = `110^{o}`
b, Do Om là tia pg $\widehat{cOb}$ nên
$\widehat{mOb}$ = $\widehat{cOb}$/2 = `110^{o}`/2 = `55^{o}`