面积公式：S=1/2absinCa=2S/(bsinC)=2*6/(4*sinπ/4) = 12/(2√2) = 3√2余弦定理：c²=a²+b²-2abcosC = (3√2)²+4²-2*3√2*4*√2/2 = 10c=√10
