Answer:
a 2-a1=18-9=9
a3-a2=27-18=9
a4-a3=36-27=9
a5-a4=45-36=9
a6-a5-54-45-9
a7-a6=63-5459
a8-a7=72-63=9
a9-a8=81-72=9
a10-a9-90-81-9
The difference between every two adjacent members ofthe series is constant and equal to 9
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