1/f+1/90-f=1/20 решите уравнение

Question

1/f+1/90-f=1/20 решите уравнение

Answer 1

1 / (90 - f)·f = 1/20

(90 - f)·f = 20

90·f - f²-20 = 0

D = 8100 - 4·90·20 = 900

f₁ = (90 - 30) / 2 = 30

f₂ = (90 + 30 ) / 20 = 60