If 3rd and 10th terms of an A.P. is 9 and 21. Then find the sum of first 12 terms
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4
Answer:
180
Step-by-step explanation:
A.T.P.,
a+2d=9
a+9d=21
Solving simultaneously,
a+9d=21
a+2d=9
Subracting, we get 7d=12
d=12/7
a=21-9*12/7
=21-108/7
=(147-108)/7
=39/7
sum of 12 terms=6(78/7+132/7)
=6*30
=180
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