Should the plural be z = a + bi or Z = a + IB I saw z = a + bi in my high school math textbook But the advanced mathematics book says z = a + IB Which one is right

Should the plural be z = a + bi or Z = a + IB I saw z = a + bi in my high school math textbook But the advanced mathematics book says z = a + IB Which one is right

It should be Bi, I is an identifier, but the meaning is the same