Question

How can I get its answer. Please explain me and do not copy it from Google please.​

Answer 1

Answer:

yu may do it like this

sum(9) = 1 + 2 + 3 + 4 ........... + 9

= 9*10/2

= 45

sum(99) = 45 + (10 + 45) + (20 + 45) + ..... (90 + 45)

= 45*10 + (10 + 20 + 30 ... 90)

= 45*10 + 10(1 + 2 + ... 9)

= 45*10 + 45*10

= sum(9)*10 + 45*10

sum(999) = sum(99)*10 + 45*100