Question

what is the focal length of a double concave lens kept in air with two spherical surfaces of radii R1=30cm and R2=60cm. Take refractive index of lens as n =1.5. plzzz i want it know

Answer 1

I think so the answer might be like this way i am not sure
we know that ------    F = R by  2
where R is radius  and F = focal length 
 therefore      first concave lens it must be 
F = 30 by 2
ie = 15 cm 
and the second 
F =60 by 2
ie = 30 cm  

Answer 2

R1=-30,R2=60,and also given lens is

in air so n=1.5

using 1/f=(1/R1-1/R2)

1/f=(1.5-1)[1/(-30)-1/60]

After solving

F=-40