Review the proof of cos(θ/2) = ±√1+cos(θ)/2 which step contains an error

Solve 3x-18=2y and 5x-6y=6 by elimination show work

If i have 20$ and i gave 5$ away how much i got

When you have 25 numbers, and jake picks 3 random numbers and puts them back, what is the chance bob has of picking those 3 numbers when he picks 6 random numbers (without putting them back)? explain.