अनुक्रम 7,77,777,........ के n पदों तक का योग ज्ञात करे
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a = 7
d = 77 - 7
= 70
An = a + ( n - 1 ) d
= 7 + ( n - 1 ) ( 70 )
= 7 + 70n - 70
= - 63 + 70n
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