What does it mean to square a value/ expression

In the proof of the triangle angle sum theorem, you must draw an auxiliary line. which construction must you use to draw this line?

Suppose i flip two identical coins. what is the probability that i get one head and one tail?

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.