A=90, A>B>C, 角C最小.
sinA+sinC=2sinB
1+sinC=2sin(90-C)
1+sinC=2cosC
sin^2 C=(1-cos^2 C)=(2cosC-1)^2=4cos^2 C-4cosC+1
5cos^2 C-4cosC=0\
cosC=4/5