Question

Two thin convex lenses are in contact and the focal length of the combination is 80 cm.If the focal length of one lens is 10 cm, then the power of the other lens will be

Answer 1

1/f = 1/f1+ 1/f2
1/80 = 1/10+1/f2
1/80-1/10 =1/f2
-7/80 = 1/f2
f2 = -80/7 = -11.4cm