How do magnification and focal length change for a combination of thin lenses ?

Derive the expression for the equivalent focal length of a combination of two thin convex lenses in contact.

Obtain an expression for focal length of a combination of thin lenses in contact.

Combination of lenses are used in optical instruments to obtain the required power. Obtain an expression for the effective focal length of the combination of two thin convex lenses in contact.

Obtain the equivalent focal length of combination of thin lenses placed in contact,

Obtain an expression for the combined focal length for two thin convex lenses kept in contact and hence obtain an expression for the combined power of the combination of the lenses.

Obtain the equation of magnification of combination of lenses.