1. A ________ number has the form a + bi, where a ≠ 0, b = 0.
2. An ________ number has the form a + bi, where a ≠ 0, b ≠ 0.
3. A ________ ________ number has the form a + bi, where a = 0, b ≠ 0.
4. The imaginary unit i is defined as i = ________, where i2 = ________.
5. When a is a positive real number, the ________ ________ root of −a is defined as √−a = √ai.
6. The numbers a + bi and a − bi are called ________ ________, and their product is a real number a2 + b2.