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प्रश्न
Find the sum to n terms 3 + 33 + 333 + 3333 + …
उत्तर
Sn = 3 + 33 + 333 +… upto n terms
= 3(1 + 11 + 111 + … upto n terms)
= `3/9` (9 + 99 + 999 + ... upto n terms)
= `3/9` [(10 − 1) + (100 − 1) + (1000 − 1) + ... upto n terms]
= `3/9` [(10 + 100 + 1000 + ... upto n terms) − (1 + 1 + 1 + ... n times)]
But 10, 100, 1000, … n terms are in G.P.
with a = 10, r = `100/10` = 10
∴ Sn = `3/9[10((10^"n" - 1)/(10 - 1)) - "n"]`
Sn = `3/9[10/9(10^"n" - 1) - "n"]`
∴ Sn = `3/81[10(10^"n" - 1) - 9"n"]`
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