To Be, or Not to Be...

Thunder King Rai-Oh (Thunder-Type/LIGHT/Level 4/ATK 1900/DEF 800)

Neither player can add cards from their Deck to their hand except by drawing them. You can send this face-up card to the Graveyard to negate the Special Summon of 1 of your opponent monsters, and destroy it.

Thunder King Rai-Oh CAN negate Special Summons that DO NOT start a Chain. 

Thunder King Rai-Oh CANNOT negate Special Summons that DO start a Chain. 

Formula Synchron, given the right circumstances, can fall into either category.

Formula Synchron (Machine-Type/LIGHT/Level 2/ATK 200/DEF 1500)
1 Tuner + 1 non-Tuner Monster

When this card is Synchro Summoned, you can draw 1 card. During your opponent's Main Phase, you can Synchro Summon using this face-up card you control as a Synchro Material Monster.

For example, if you have a face-up Formula Synchron and Caius the Shadow Monarch on the Field, I CANNOT negate your Synchron Summon with Rai-Oh during my turn, but I CAN during yours.

Make sense?

Remember players, the game ends with you.