Giải thích các bước giải:

Vì $Oc$ nằm giữa $Oa, Ob$

$\to \widehat{aOc}+\widehat{cOb}=\widehat{aOb}=40^o$

Vì $7\widehat{aOc}=4\widehat{bOc}\to \widehat{bOc}=\dfrac74\widehat{aOc}$ 

$\to \widehat{aOc}+\dfrac74\widehat{aOc}=40^o$

$\to \dfrac{11}4\widehat{aOc}=40^o$

$\to \widehat{aOc}\approx 14.54^o$