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?

A parallelogram has three consecutive vertices (-3, 4), (0, -4) and (5, 2). The fourth vertex is

If the lines y + px = 1 and y - qx = 2 are perpendicular, then which one of the following is correct?