因为a＋c＝根2 ＊b所以由正弦定理，得sinA+sinC=√2 sinB 又因为A－C＝90度，所以sin(90+C)+sinC=√2 sin(180-C-90-C)sinC+cosC=√2 cos2Csin(C+45)=sin(90-2C)所以C=15°
