Đáp án`+`Giải thích các bước giải:

`a,`Theo định lý cosin trong `\Delta AOB` thì:

`AB^2 = OA^2 + OB^2 -2.OA.OB.cos hat(AOB)`

`=> (4sqrt(2))^2 =4^2 +4^2 -2.4.4. cos hat(AOB)`

`=> hat(AOB)=90^@`

`=> l`$\mathop{AB_n}\limits^{\displaystyle\frown}$`=(pi.R.n^@ )/(180^@ )=(pi.4.90^@ )/(180^@ )=2pi~~6,28``(` cm `)`

`=> l`$\mathop{AB_l}\limits^{\displaystyle\frown}$`=(pi.R.n^@ )/(180^@ )=(pi.4.(360^@ -90^@) )/(180^@ )=6pi~~18,85``(` cm `)`

`b,`$\rm S_{quạt}$$\mathop{AOB}\limits^{\displaystyle\frown}$ `= (l.R)/2 = (2pi.4)/2 =4pi~~12,57``( cm^2 )`