Use the lcd to determine which fraction is greater 13/18,16/21

Can someone me and do the problem plz so i can understand it more better

Ana drinks chocolate milk out of glasses that each holdof a liter. she has of a liter of chocolate milk in her refrigerator. 10 how many glasses of chocolate milk can she pour?

If cos(2x) = tan^2(y), show that cos(2y) = tan^2(x).