Question

if focal length of concave mirror is 20cm and v is 80cm.find u

Answer 1

1/u + 1/v = 1/f

1/v - 1/f = - 1/u

1/80 - 1/20 = - 1/u

(1 - 4)/80 = - 1/u

-3/80 = - 1/u

u = 80/3

u = 26.666 cm

Answer 2

Given–

f=20cm
v=80cm

We know that

$\frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\ \frac{1}{20} = \frac{1}{u} + \frac{1}{80} \\ \frac{1}{20} - \frac{1}{80} = \frac{1}{u} \\ \frac{1}{u} = \frac{1}{20} - \frac{1}{80} \\ \frac{1}{u} = \frac{4 - 1}{80} \\ \frac{1}{u } = \frac{3}{80} \\ u = \frac{80}{3}$
=26.66cm