What is the common difference between successive terms 9,2.5,-4,-10.5,-

The common difference between this series of numbers is 6.5. In other words 9-2.5 is 6.5, 2.5 and -4 shows a common difference of 6.5, -4 and -10.5 difference is still 6.5 (-6.5) and -10.5 t0 -17 is -6.5 so even where there are plus and minus numbers they are such that the absolute difference is always 6.5.

Avertical or horizontal line used in graphs is called a(n) equatorhope this

Yes it does it sure does me later