前>>880
>>887
AB=2,BC=3,CA=4,CD=5,AD=6,BD=7のとき、
余弦定理より、
cos∠ADB=(36+49-4)/(2・6・7)=81/84=27/28
=0.96428571428571……
=cos15.358885580°
cos∠BDC=357=(25+49-9)/(2・5・7)=65/70=13/14
=0.9285714285714……
=cos21.786789298°
cos∠CDA=456=(25+36-16)/(2・5・6)=45/60=3/4
=0.75
=cos41.409622109°
∴∠ADB+∠BDC+∠CDA=15.358885580°+21.786789298°+41.409622109°
=78.555296987°
≒78.6°