CosC=3√ 10/ 10 According to the cosine theorem, then when a, C∈(0, π), sinc = √10, Sina = √ 5/5.
cos2C=2cos？ C- 1=4/5
cosB=cos(π-A-C)=-cos(A+C)
=sinAsinC-cosAcosC
=-√2/2
When B∈(0, π), B=3π/4.
2. Sina: sinc = a: c = √ 5/5: √10/√ 2:1.
A-c=√2- 1, so a=√2, c= 1.
S△ABC=？ sinBac=？ ×√2/2 ×√2× 1= 1/2