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anonymous
 5 years ago
please help
The 11th term of an arithmetic progression is 1. the sum of the first 10 terms is 120.
find the 4th term :S
formula for the...
sum of terms=1/2n(a+l)=1/2n{2a+(n1)d}
thanks
anonymous
 5 years ago
please help The 11th term of an arithmetic progression is 1. the sum of the first 10 terms is 120. find the 4th term :S formula for the... sum of terms=1/2n(a+l)=1/2n{2a+(n1)d} thanks

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anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0\[{11(a_1 + a_{11}) \over 2} = 120 + 1 \]

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0so\[{a_1 + 1} = 22\] thus a_1 = 21

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0Then you can say\[a_{11} = a_1 + 10d\] \[a_11 = 1 = 21 + 10d\]

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0that means that the 4th term is 21+3(2) = 15

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0but you didnt even use 4 once :S??? and i dont really understand where 21+3(2)=15 comes from :(

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0ok, \[a_n = a_1 + d(n1)\] so\[a_4 = a_1 + (2)(41) = 21 + 3(2)\]

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0the idea of arithmetic sequence is that you are "Adding the same number for all sequence"

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0sorry but i first thought that i knew where 11(a1+a11)2=120+1 came from but i dont :S

anonymous
 5 years ago
Best ResponseYou've already chosen the best response.0the first ten numbers add up to 120, so 10(a1+a10) /2= 120 now if you add a11 to it, it is 120 + 1 but adding a11 to the sum from a1 to a10 is as same as adding from a1 to a11 so 11(a1 + a11)/2 = 121
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