分别用while,do while,for语句计算，并输入如下算式前20项的和，结果保留2位小数。1-1/2+1/3-1/4+1/5…

为了计算这个算式的前20项和并保留两位小数，我们可以分别使用`while`、`do-while`和`for`循环结构。这里是一个简单的示例，我们将使用Python语言编写：

# Using while loop
total_while = 0
i = 1
while i <= 20:
    term = 1 / i if i % 2 != 0 else -1 / i
    total_while += term
    i += 1
sum_while = round(total_while, 2)

# Using do-while loop (Python不直接支持，需要转换思路)
i = 1
sum_do_while = None
while True:
    term = 1 / i if i % 2 != 0 else -1 / i
    total = sum_do_while or 0
    total += term
    sum_do_while = total
    i += 1
    if i > 20:
        break
sum_do_while = round(sum_do_while, 2)

# Using for loop
total_for = 0
for i in range(1, 21):
    term = 1 / i if i % 2 != 0 else -1 / i
    total_for += term
sum_for = round(total_for, 2)

print("While loop result:", sum_while)
print("Do-while (modified) result:", sum_do_while)
print("For loop result:", sum_for)

每个循环会计算出前20项的和，然后使用`round()`函数将结果保留两位小数。