An algebra problem by A Former Brilliant Member
Find the sum of the products of the eighteen numbers ± 1 , ± 2 , ± 3 , ± 4 , ± 5 , ± 6 , ± 7 , ± 8 a n d ± 9 taking two at a time
The answer is -285.
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2 solutions
Sir I have made the required correction
( 1 − 1 + 2 − 2 . . . . . . . + 9 − 9 ) 2 = 1 2 + 1 2 + 2 2 + . . . . . . . . + 9 2 + 9 2 + 2 S implies 0 = 2 ( 1 2 + 2 2 + . . . . . + 9 2 ) + 2 S thus S = − 2 8 5
There's 18 numbers, not 20.