139. The sum upto infinity of the series sqrt(2) 1 / (sqrt(2)) 1 /...
Solution:
Given, the series is: sqrt(2) 1 / (sqrt(2)) 1 / (2sqrt(2)) ...
Let Sn be the sum of the first n terms of the series.
Then, S1 = sqrt(2), S2 = sqrt(2) + 1 / sqrt(2), S3 = sqrt(2) + 1 / sqrt(2) + 1 / (2sqrt(2)), ...
To find the sum of the series, we need to take the limit as n approaches infinity of Sn.
Step 1: Finding a pattern
Let's try to find a pattern in the series by writing out the first few terms:
S1 = sqrt(2)
S2 = sqrt(2) + 1 / sqrt(2)
S3 = sqrt(2) + 1 / sqrt(2) + 1 / (2sqrt(2))
S4 = sqrt(2) + 1 / sqrt(2) + 1 / (2sqrt(2)) + 1 / (4sqrt(2))
From this, we can see that the nth term of the series is given by:
1 / (2^(n-1) * sqrt(2))
Step 2: Finding the sum of the series
Using the formula for the nth term, we can write the sum of the series as:
Sn = sqrt(2) + 1 / sqrt(2) + 1 / (2sqrt(2)) + ... + 1 / (2^(n-1) * sqrt(2))
Multiplying the numerator and denominator of each term by sqrt(2), we get:
Sn = (sqrt(2) * sqrt(2) + 1 + 1 / 2 + ... + 1 / (2^(n-1))) / sqrt(2)
Now, the sum of the series in the numerator is a geometric series with first term 1 and common ratio 1/2. Thus, the sum of this series is:
1 / (1 - 1/2) = 2
Substituting this back into the expression for Sn, we get:
Sn = (sqrt(2) * sqrt(2) + 2) / sqrt(2) = sqrt(2) + 2
Step 3: Taking the limit as n approaches infinity
Finally, taking the limit as n approaches infinity, we get:
lim (n->inf) Sn = lim (n->inf) (sqrt(2) + 2) = sqrt(2) + 2
Therefore, the sum of the series is sqrt(2) + 2.
Hence, the correct option is (b) 2.
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