Question

what is the total number of four digit number

Answer 1

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There are total 9000 four digit numbers.

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Answer 2

Answer:

49495500

Step-by-step explanation:

here the four digit number are

here the four digit number are1000, 1001, 1002, ..., 9998, 9999

$a _{1} = 1000 \\ a_{2} = 1001 \\ a _{3} = 1002 \\ a _{n} = 9999$

$d = a _{2} - a _{1} = 1001 - 1000 = 1 \\ d = 1$

here, n = ?

here, n = ?to find n

$a _{n} = a + (n - 1) \times d$

$9999 = 1000 + (n - 1) \times 1 \\ 9999 - 1000 = n - 1 \\ 8999 =n - 1 \\ n = 1 + 8999 \\ n = 9000$

now,

$S _{n} = \frac{n}{2} (2a + (n - 1) \times d$

$S _{9000} = \frac{9000}{2} (2(1000) + (9000 - 1) \times 1) \\S _{9000} = 4500(2000 + 8999) \\ S _{9000} = 4500(10999) \\ S _{9000} = 49495500$

thus, the sum of the 4 digit number is 49495500

second answer the total number of four digit number is 9000