This is an easy problem in my school exams that killed most of my friends. Probably they are much concentrated on harder problems. Do you remember the basics?
S n = ( 1 ) ( 3 ) + ( 2 ) ( 4 ) + ( 3 ) ( 5 ) + . . . + n ( n + 2 )
What is the last three digit of S 3 0 ?
You may use a calculator.
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The good old basic ( a + b ) ( a − b ) = a 2 − b 2 , where b is half of the difference between the two numbers.
splendid.........
u indeed have a genius brain
Beautiful question
just open the bracket, n(n+2)
n 2 + 2n
apply the summition
∑ i = 0 n n 2 + ∑ i = 0 n 2n
∑ i = 0 n n 2 + 2 ∑ i = 0 n n
6 n ( n + 1 ) ( 2 n + 1 ) + 2 2 n ( n + 1 )
Now just put n=30
you will get 10385
so the ans is 3 8 5
Was that "you can use a calculator" lust to trick us??
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The calculator is not much important though... Just some people like me use calculator to multiply 3 0 × 3 1 × 6 1 haha
Lol ... just summation of n^2 + 2n from n=1 to 30 ...
1 | 3 | |||
2 | 11 | 8 | ||
3 | 26 | 15 | 7 | |
4 | 50 | 24 | 9 | 2 |
5 | 85 | 35 | 11 | 2 |
a ⋅ 1 3 + b ⋅ 1 2 + c ⋅ 1 + d = 0 3
a ⋅ 2 3 + b ⋅ 2 2 + c ⋅ 2 + d = 1 1
a ⋅ 3 3 + b ⋅ 3 2 + c ⋅ 3 + d = 2 6
a ⋅ 4 3 + b ⋅ 4 2 + c ⋅ 4 + d = 5 0
a = 3 1 , b = 2 3 , c = 6 7 , d = 0
a n = 3 1 n 3 + 2 3 n 2 + 6 7 n
a n = 6 n ( 2 n 2 + 9 n + 7 )
a 3 0 = 1 0 3 8 5
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Note that n ( n + 2 ) + 1 = ( n + 1 ) 2
S n = 1 ∗ 3 + 2 ∗ 4 + 3 ∗ 5 + … + n ∗ ( n + 2 )
⇒ S n = 1 ∗ 3 + 1 − 1 + 2 ∗ 4 + 1 − 1 + … + n ∗ ( n + 2 ) + 1 − 1
⇒ S n = 2 2 − 1 + 3 2 − 1 + … + ( n + 1 ) 2 − 1
⇒ S n = 2 2 + 3 2 + … + ( n + 1 ) 2 − n
⇒ S n = 1 2 + 2 2 + 3 2 + … + ( n + 1 ) 2 − ( n + 1 )
⇒ S n = 6 ( n + 1 ) ( n + 2 ) ( 2 n + 3 ) − ( n + 1 )
Putting in n = 3 0
S 3 0 = 1 0 4 1 6 − 3 1 = 1 0 3 8 5
Therefore answer 3 8 5