12th grade
posted by chris .
if cos Q=5/9, find 5/9

5/9
arccos 5/9 is quite another thing, it is equal to 56.25 deg 
cos(Q)=5/9
cos inverse both sides
cos^1(cos(Q))=cos^1(5/9)
cos^1(cos(Q))=Q, based on inverse properity
Q=cos^1(5/9)