moodyshaliyah01
24.10.2019 •
Mathematics
Find the sum of the first 25 terms. 2,8,14,20
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Ответ:
a(n)=a+d(n-1), a=initial term, d=common difference, n=term number.
We know that a=2 and d=(20-14)=(14-8)=(8-2)=6 so
a(25)=2+6(25-1)
a(25)=146
The sum of an arithmetic sequence is the average of the first and last terms times the number of terms, in this case:
s(25)=25(2+146)/2
s(25)=1850
And if you care to remember the arithmetic sum formula it is:
s(n)=(2an+dn^2-dn)/2, using a=2, d=6, and n=25 we get:
s(25)=(4(25)+6(25^2)-6(25))/2
s(25)=1850
Ответ:
Add 120 and 140 which gives you 360 and divide by 2 since there is 2 numbers which gives you 180 :)