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Why?

If you takea2 - a1...

0 , 3 , 6 , 9 , 12 , ...  3 - 0 = 3 ... You get the difference ( 3 )

Same witha3 - a2...

0 , 3 , 6 , 9 , 12 , ...  6 - 3 = 3 ... the difference

Sometimes, it's not so easy to find this critter!

Check out this sequence:

( 2 / 5 ) , ( 19 / 10 ) , ( 17 / 5 ) , ( 49 / 10 ) , ( 32 / 5 ) , ...

Can't tell by looking, can ya?

So, let's do the official difference hunt:

a2 - a1 = ( 19 / 10 ) - ( 2 / 5 ) = ( 3 / 2 )
This is probably the difference, but
we'd better check it!

a2 + ( 3 / 2 )better get us up to17 / 5!

a2 + ( 3 / 2 ) = ( 19 / 10 ) + ( 3 / 2 ) = ( 17 / 5 ) ... yep!