Day 3:Sum of the first nth term of Series

Details:

Your task is to write a function which returns the sum of following series upto nth term(parameter).
Series: 1 + 1/4 + 1/7 + 1/10 + 1/13 + 1/16 +...

Rules:
You need to round the answer to 2 decimal places and return it as String.
If the given value is 0 then it should return 0.00
You will only be given Natural Numbers as arguments.

Examples:
SeriesSum(1) => 1 = "1.00"
SeriesSum(2) => 1 + 1/4 = "1.25"
SeriesSum(5) => 1 + 1/4 + 1/7 + 1/10 + 1/13 = "1.57"

My Solution:

def series_sum(n):
    sum = 0.0
    for i in range(n):
        sum += float(1) / ((i)*3+1)
    sum = round(sum, 2)
    return format(sum, `.2f`)

Best Practice:

def series_sum(n):
    return `{:.2f}`.format(sum(1.0/(3 * i + 1) for i in range(n)))

Tips:

1. 除法保留兩位小數用round(a/b, 2)。
2. 小數轉成字串保留兩位小數寫成`%.2f`%num