Route 9,10,11

Route 11

Go back through the cave and go east when you get out. You'll be on Route 11. There are many trainers, many items, and a whole lot of grass. When you reach the gate, one of Prof. Oak's aides will be upstairs. If you have caught up to thirty Pokemon, he'll give you an Item Finder. This item helps you detect hidden items that cannot be seen. There's also another trainer that'll trade you a Nidorina for a Nidorino. You can't go to the other side since there's a Snorlax blocking the way. Now go back to Cerulean City and cut the tree that was blocking the way. Then you'll get onto Route 9.

Locations (Route 11)
Drowzee Ekans Sandshrew Spearow

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Route 9

On Route 9 you'll be battling many trainers. Here, you'll find TM40 - Aerial Ace. To reach the end of this route, just keep on going east, where you'll arrive at Route 10.

Locations (Route 9)
Ekans Sandshrew Rattata Spearow

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Route 10

Route 10 is short for now. You'll see some water. When your Pokemon learns Surf, it can surf to the Power Plant, just south of Route 10. Make sure that you have taught one of your Pokemon the Flash HM in order to enter Rock Tunnel.

There's a Pokemon Center for you to heal your Pokemon here. One of Prof. Oak's aides is here. If you've caught at least twenty Pokemon, he'll give you the Everstone. Let your Pokemon hold the Everstone, and they won't evolve.

Locations (Route 10)
Ekans Sandshrew Spearow Voltorb