Question

How are each term related to one another in the sequence 2,90,4,87,6,12,8,51,10 ?​

Answer 1

Every other number from the first, 3rd, 5th, ..etc it adds 2 for the next number.
For the 2nd, 4th, 6th, etc term, its supposed to be 90, 78, 65, 51?
If so, It started with -12 then -13 after that -14 find the next term, all you need to do is to -15 to the current number, which is 51. then you get 36