How to add some value between two datas in an array?

i want to ask how to put some value between two datas like "interp" function. its because "interp" function only work for at least 9 input datas.

i want to do something like "interp" function but with just 2 input datas

for example

i have A=[1 2]

then become A1=[1 1.5 2 2.5]

[KSSV]

A1 = [1 1.5 2] 

The above canbe done using interp1 and the result is trusted. This ocmes under inteprolation.

A1=[1    1.5    2    2.5]

The above can alos be done using interp1 but the result for 2.5 cannot be trusted, as it comes under extrapolation.

[Rik]

If you want a linear interpolation you could use polyfit (or just determine the coefficients yourself).