AP Average of n numbers of arithmetic progression (AP) is the average of the smallest and the largest number of them. The average of m number can also be written as x + d(m-1)/2.# P. f# F3 g1 p& T9 K8 c' `
Example:9 N4 r& R5 M; Z; l5 k: r
The average of all integers from 1 to 5 is (1+5)/2=3
$ e. p: b6 g9 m( j X5 W+ ^ The average of all odd numbers from 3 to 3135 is (3+3135)/2=15694 J4 I- c9 ?; H
The average of all multiples of 7 from 14 to 126 is (14+126)/2=707 K6 n* {$ Y5 a% B
remember:
$ t8 M: [9 i2 \# ]8 e4 o/ S Make sure no number is missing in the middle.
6 I3 x, B! E5 \% _1 H$ \; A' u# y With more numbers, average of an ascending AP increases.
1 s, S: B! |1 ~( s9 h With more numbers, average of a descending AP decreases.* [% b3 T+ s, F
AP:numbers from sum
) V+ v9 {7 x* C: W% } given the sum s of m numbers of an AP with constant increment d, the numbers in the set can be calculated as follows:
% L9 u& ^& t/ c" K1 X the first number x = s/m - d(m-1)/2,and the n-th number is s/m + d(2n-m-1)/2.
: ^$ G' ]1 W4 W- ?3 L/ z Example:
/ _9 e- k ~2 Q \/ e5 ?+ P4 K if the sum of 7 consecutive even numbers is 70, then the first number x = 70/7 - 2(7-1)/2 = 10 - 6 = 4.
& [1 T b, H! y8 h% G the last number (n=m=7)is 70/7+2(2*7-7-1)/2=10+6=16.the set is the even numbers from 4 to 16.
" C9 ~7 }* [ n8 p Remember:7 V, a4 q J' g1 X( u1 X' e
given the first number x, it is easy to calculate other numbers using the formula for n-th number: x+(n-1)2 Q8 @, Y {9 i8 t0 l1 ]
AP:numbers from average9 P! q8 S- s4 `
all m numbers of an AP can be calculated from the average. the first number x = c-d(m-1)/2, and the n-th number is c+d(2n-m-1)/2, where c is the average of m numbers.
' \) P1 K6 r) P# P3 r6 l- K7 k Example:* j) m1 {0 n# ?. @! v/ i$ O
if the average of 15 consecutive integers is 20, then the first number x=20-1*(15-1)/2=20-7=13 and the last number (n=m=15) is 20+1*(2*15-15-1)/2=20+7=27.
' [& D ^ t5 A. c4 \ if the average of 33 consecutive odd numbers is 67, then the first number x=67-2*(33-1)/2=67-32=35 and the last number (n=m=33) is 67+2*(2*33-33-1)/2=67+32=99.* r% v% R0 i+ a
Remember:; e8 u4 X" h6 n' @2 Z
sum of the m numbers is c*m,where c is the average.
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