31.3.2 Multiplication: Just Substitute i²=−1
Core ideaPicture this: multiply it out like expanding brackets, and every time you hit i² swap in −1, and naturally you land on (ac−bd)+(ad+bc)i
Module goal. We build the rules for adding, subtracting, multiplying, and dividing — learning both to grind through them algebraically and to see what addition and subtraction mean geometrically