bài 6 ặ ????????????

$\color{cadetblue}{\large{@Sandy}}$

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$\color{darkgoldenrod}{a)}$  $\widehat{AOB}\ \ \text{là góc ở tâm chắn}\ \overparen{AmB}$

$\Rightarrow\ \widehat{AOB}=sđ\ \overparen{AmB}$

$\text{Mà}\ sđ\ \overparen{AmB}=100^\circ\ \text{(gt)}$

$\Rightarrow\ \widehat{AOB}=100^\circ$

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$\color{darkgoldenrod}{b)}$  $\widehat{AMB}\ \ \text{là góc có đỉnh ở ngoài đường tròn chắn}\ \overparen{AmB}\ \text{và}\ \overparen{CnD}$

$\Rightarrow\ \widehat{AMB}=\dfrac{1}{2}(sđ\ \overparen{AmB}-sđ\ \overparen{CnD})$

$\Leftrightarrow\ 40^\circ=\dfrac{1}{2}(100^\circ-sđ\ \overparen{CnD})$

$\Leftrightarrow\ 80^\circ=100^\circ-sđ\ \overparen{CnD}$

$\Leftrightarrow\ sđ\ \overparen{CnD}=20^\circ$