Two lenses of power $+12 \ D$ and $-2 \ D$ are placed in contact. The combined focal length of the combination will be ....... $cm$.

$A$ convex lens of focal length $84 \ cm$ is in contact with a concave lens of focal length $12 \ cm$. The power of the combination (in diopters) is:

Two lenses of power $3\,D$ and $-5\,D$ are combined to form a compound lens. An object is placed at a distance of $50\,cm$ from this combination. At what distance (in $cm$) will the image be formed?

$A$ convex lens has $9 \ cm$ focal length and a concave lens has $-18 \ cm$ focal length. The focal length of the combination in contact will be.....$cm$

$A$ convex lens of focal length $30\, cm$ and a concave lens of focal length $10\, cm$ are placed coaxially at a distance $d$ apart. Parallel rays are incident on the convex lens. The rays emerging from the concave lens are also parallel. Then $d = $ . . . . . . $cm$.

If a convex lens of focal length $80 \;cm$ and a concave lens of focal length $50 \;cm$ are combined together,what will be their resulting power?