Puzzles - Missing letters puzzles

Split the circle into 4 quarters, horizontally and vertically, with 2 segments each. Starting with the top left segment, the sum of the numerical values of the letters in the 2 segments equals 15. The sum of the next pair of segments clockwise equals 16, then 17, then 18.

Taking each pair of letters in each row, the numerical value of one letter equals the inverse numerical value of the other letter.

Start on the top left of the diagram, and move clockwise in a spiral towards the centre. Letters advance through the alphabet in steps of 5.