3. Find the 20th term of the AP 9, 13, 17, 20
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Ap:9,13,17,20,....
a=9
d=a²-a¹
=13-9
d=4
n=20
an=a+(n-1) d
a20=9+(20-1)4
=9+19×4
=9+76
a20 =85.
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