Решите систему:
f+d=120
1/f+1/d=1/25

f+d=120 1/f+1/d=1/25 f+d/fd=1/25fd=120*25f=120-d120d-d^2=3000d^2-120d+3000=0D=14400-12000=2400d12=(120+-√2400)/2f12=120-(120+-√2400)/2