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Along with your ridge vent, I would add two large box vents. One at either end of the hip ridge.
Or you could add a power vent for those hot summer days to exhaust some of the excess heat.

For more info

http://roofingcontractorreview.com/Hip-Roof-Venting.html

and

http://roofingcontractorreview.com/Powered-Attic-Fans.html