Answer 1

Let z = x + iy, then

`|("z" + 3"i")/("z" - 6"i")|` = 1 gives

`|(x + iy + 3"i")/(x + iy - 6"i")|` = 1

∴ `|(x + (y + 3)"i")/(x + (y - 6)"i")| = 1   ...[because |"z"_1/"z"_2| = |"z"_1/"z"_2|]`

∴ |x + (y + 3)i = |x + (y – 6)i|

∴ `sqrt(x^2 + (y + 3)^2) = sqrt(x^2 + (y - 6)^2)`

x² + (y + 3)² = x² + (y – 6)²

∴ x² + y² + 6y + 9 = x² + y² – 12y + 36

∴ 18y – 27 = 0

∴ 2y – 3 = 0

This is the equation of the required locus.