a,ta co $\widehat{AMI}=\widehat{AHI}=90^0=>$ dpcmb,tu giac AMHI noi tiep duong tron ban kinh $\frac R2$;duong kinh AI =>$(O) va (I) tiep xuc trong$
c,xet tu giac AMIN co $\widehat{ANI}=\widehat{AMI}=\widehat{MAN}=90^0$=>dpcm
d;ta co $\widehat{AIB}=120^0=>$$S=\pi\frac{120^0R^2}{360^0}-\frac{AB.R.cos60}{2}=\pi\frac{R^2}2-\frac{\sqrt3R^2}4$