3, 9, 21 , 45-----

is this a arthemetic sequence? how?

Answer:
3 , 9 , 21 , 45 .....
We know if this sequence is Arithmetic sequence than the common difference must be same for any two consecutive digits :
9 - 3 = 6
And
21 - 9 = 12
We can see that are not equal so we can say that sequence is not a arithmetic sequence .
But we can find out its next digit As:
3  + 6 = 9
9 + 12 = 21
21 + 24 = 45
So next digit is As
45 + 48 = 93                                    

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