先根据正弦定理,求出角B.5/sinπ/4=5/sinB
解得B=π/4所以CosB=Cos45°=√2/2
由5√2/3/sinB=5/sin45º
sinB=1/3
故cosB=±2√2/3
∵a∶sinA=b∶sinB
∴sinB=bsinA/a=[5√2/3×﹙√2/2﹚]/5=1/3
∵a>b
∴B<A
∴cosB=√﹙1-sin²B﹚=√[1-﹙1/3﹚²]=2√2/3
a>b
A>B
A+B<π/2
C>π/2
正弦定理
sinB=sinA*b/a
=sin(π/4)×(5√2/3)/5
=√2/2×(5√2/3)/5
=1/3
cosB=√(1-(1/3)²)
=2√2/3