根据余弦定理a²=b²+c²-2bccosA代入得
300²=400²+650²-2*400*650cosA
90000=160000+422500-520000cosA
520000cosA=160000+422500-90000
520000cosA=492500
cosA=0.94711538461538461538461538461538......
∠A=18.716950657371325634464155357702......°
=18°43.01703944227953806784932146212......'
=18°43‘1.0223665367722840709592877271962......“
≈18°43‘1“
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