My question is what is the formulas, for combining runes? How do you know which ones you are going to get and is it worth to combine them?????

2 of the same rank make a random one of the same rank. 5 of a rank 1/2 make 1 random rank 2/3. (5 make one rank higher)

It's more of a risk, it is the cheapest way to get some t3 runes but you're usually not gonna get the one you want, however combining is the only way to get tier 3 runes when you aren't high enough lvl.

When I want to upgrade my runes, I'll get the cheapest ones of that tier available and throw them in. Probably won't be what you want, but hey, gives you a bit of variety. And 75 IP for a random tier 2 rune isn't horrible.

It's totally worth it if you have Tier 1 or 2 runes. Once you hit level 20 though you should combine to upgrade all your runes to Tier 3, and then combine the useless ones in hopes of getting something you need, until you have all the Tier 3 runes you need. After you've done this, the combiner is pretty pointless.