a/sinA=c/sinC
sinA=a×sin(π/4)/c
=a×(√2/2)/2a
=√2/4
cos2A=1-2sin²A=1-2×(√2/4)²=3/4
sin2A=√7/4( 由△知取正值）
cos(2A-π/3)
=coa2Acoa(π/3)+sin2Asin(π/3)
=3/4×1/2+√7/4×√3/2
=3/8+√21/8
=（3+√21）/8