I just wrote a math test and could not get that last question. It would be greatly appreciated if someone could show me what I did wrong just so that I know for next time.
Question: In an arithmetic sequence you have twenty terms. The sum of the 20 terms is 1590. Term 8 is 62. How many of the twenty terms are below 400?
I wrote;
a= ?
d=?
t8= 62 sn= n(a+tn)/2
1590= 62(a+8)/2
a= 43.29...
(which I rounded to 43). I know know what the swequence starts with.
I then inputed everything into the arithmetic formula tn= a+(n1)d and solved for d (which i got 2.7) then I concluded that term 20 was about 94 so all twenty of the terms were under 400. I know it's completely wrong though!
Question: In an arithmetic sequence you have twenty terms. The sum of the 20 terms is 1590. Term 8 is 62. How many of the twenty terms are below 400?
I wrote;
a= ?
d=?
t8= 62 sn= n(a+tn)/2
1590= 62(a+8)/2
a= 43.29...
(which I rounded to 43). I know know what the swequence starts with.
I then inputed everything into the arithmetic formula tn= a+(n1)d and solved for d (which i got 2.7) then I concluded that term 20 was about 94 so all twenty of the terms were under 400. I know it's completely wrong though!