範例9

設DABC之三頂點為A(2, 3), B(5, 7), C(9, 4), 試求三內角.

[解法]

<Sol.>

 cosA=



=cos45°
ÐA=45°
‚ cosB=


=0
=cos90°
ÐB=90°

ƒÐC=90°-ÐA=90°-45°=45°