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If I play lets say a artifact with CMC 1 and there is two Sphere of Resistance in play and a Trinisphere. Do you have to pay 5 mana or just 3?

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You have to pay just 3. From a ruling on Trinisphere, emphasis added:

12/1/2004: Trinisphere's ability affects the total cost of the spell. It is applied after any other cost increasers or cost reducers are applied: First apply any cost increases. Next apply any cost reducers. Finally look at the amount of mana you have to pay. If it's less than three mana, you'll pay three mana.

So, after taking into account all the present Spheres of Resistance, we determine whether the spell you're casting costs less than 3 mana. Your artifact's already been beefed up to costing {3}, so Trinisphere does nothing extra to it and remains costing {3}.


The rules basis for this is in (currently) Casting Spells, 601.2f. Emphasis added again:

601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can’t be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.

In that bolded section in the middle we're applying Sphere of Resistance, and any other cost increases and reductions. Then in the bolded section at the end, we apply effects like Trinisphere which just directly say what the total cost is.

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