First, tolerance d

Odd sum and reduced couple sum = a1-(n-1)/2 * d = 44-33 =11should be understandable. If you don't understand, let's use the small number 5 as an example, for example, 5432 1, s odd-s even = the first term 5-(5- 1)/2* tolerance 1.

Then the sum of a1-an =18 =-(n-1) d is brought into the above formula to get a 1=20.

So an=2

Sum =44+33=77=(a 1+an)n/2 gives n=7 gives 18=-(n- 1)d then d=-3.

The general term can be obtained:

an = a 1+(n- 1)d = 20-(n- 1)* 3