What's the exact value of cos 2pi/3?

Problem melissa is her dad build a walkway in their backyard. the walkway will be 54 feet long and 1.5 feet wide. the local hardware store sells tiles which are 1.5 by 1.5 feet and come in boxes of 12. how many boxes of tiles do they need?

The measure of angle 0 is 600 degrees

The measurement of one angle of a right triangle is 42° what is the measurement of the third angle? (there is no picture)