for any positive integer n , the sum of the first n positive integers equals n ( n + 1 ) / 2 . what is the sum of odd all the integers between 10 and 100 ?
"numbers are 11 , 13,15 - - - - - - - , 97,99 average of the set : ( largest + smallest ) / 2 = ( 99 + 11 ) / 2 = 55 ; # of terms : ( largest - smallest ) / 2 + 1 = ( 99 - 11 ) / 2 + 1 = 45 the sum = 45 * 55 = 2475 answer : c"
add(divide(subtract(subtract(100, 1), add(10, 1)), 2), 1)