Answer :

We know, nth term of an AP is
an = a + (n – 1)d
where ‘a’ and ‘d’ are first term and common difference of AP respectively
Given, third term of an A.P. is 7
a3 = 7
⇒ a + 2d = 7     (i)
the seventh term exceeds three times the third term by 2

⇒ a7 = 3a3 + 2
⇒ a + 6d = 3(7) + 2
⇒ 7 – 2d +6d = 21 + 2
⇒ 4d = 16
⇒ d
 = 4


From (i),
a = 7 – 2(4)

= -1


Also, we know sum of first 'n' terms of an AP is

Sum of first 20 terms is

= 10[-2 + 76]
= 740

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