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Why people keep posting this is beyond me.
Buying hp would solve the problem if you champion had 0 base hp... which he doesn't. Therefore, if you buy resistances, you increase your ehp by an amount proportional to the hp you already have. In other words, buying armor uses the base stats your champion already has. Buying hp doesn't do that and hence is way less effective.
If you have 1500 health and buy 30 armor you are getting 450 hp. Buying 40 armor costs 700 gold. Buying 400 hp costs 1k.