(3) Lines 1 and 2 of the answer:

To find the sum of the top 50 items of {cn}, the sequence {cn} is needed first, so {an} and {1/bn} must be found first. So bring t= 1/3 into the last line of the answer (1) and you get {an}.

{1/bn} can be obtained from the last two lines of the answer to (2).

Line 3: No explanation.

Line 4: The preceding k refers to the number of {an}, and each item in the parentheses after it indicates that there are several {1/bn} before each {an}, 0 before a 1, and1... ……ak before a2.

Line 5: According to the answer in line 4, there are only five {1/B9} left after C45.

Line 6-8: The first bracket in the equation is the sum of {an} and the last bracket is the sum of {1/bn}.

Line 9 and 10- 13: Find the sum of {an} and {1/bn} in the equations in lines 6-8, respectively.

On the lines of 14 and 15, find S45, and then add S45 to the 46th-50th digits in {{Cn}}, that is, five {1/B9}, to get S50.