Question

in cylic hydrocarbons why we write pentane as C5H10

Answer 1

Bz there are 5 carbon and they bond by sharing 2 electron that means single bond

Answer 2

Note that there is no double or triple bond in the molecule, so this is a alkane.....

Bcoz the the bonds in the end carbon atoms which should be made between them and hydrogen atoms is actually made among them, thus eliminating the formation of bond with two hydrogen atoms.
Hence we find their genral formula as Cn H2n