The third term of a GP is 3. What is the product of its first five terms?

The element in the i^th row and the j^th column of a determinant of third order is equal to 2(i + j). What is the value of the determinant?

With the numbers 2, 4, 6, 8, all the possible determinants with these four different elements are constructed. What is the sum of the values of all such determinants?

What is the radius of the circle 4x² + 4y² - 20x + 12y - 15 = 0?