Please help me with 4-6 and please answer the questions

Express the following as a function of a single angle. cos(60) cos(-20) - sin(60) sin(-20)

(b) suppose that consolidated power decides to use a level of significance of α = .05, and suppose a random sample of 100 temperature readings is obtained. if the sample mean of the 100 temperature readings is x⎯⎯ = 60.990, test h0 versus ha and determine whether the power plant should be shut down and the cooling system repaired. perform the hypothesis test by using a critical value and a p-value. assume σ = 5. (round your z to 2 decimal places and p-value to 4 decimal places.)

Which expression is a perfect cube? a. x8 b. y24 c. m28 d. x64