Students learn to solve equations using the imaginary number "i". For example, to solve x^2 + 21 = 1, first subtract 21 from both sides to get x^2 = -20. Next, take the square root of both sides to get x = plus or minus root -20. Next, think of root -20 as root -1 times root 20, which simplifies to i times 2 root 5, or 2i root 5. So x = plus or minus 2i root 5. Students also learn to square imaginary numbers, multiply imaginary numbers, and divide imaginary numbers.