SOLUTION: the 7th term of an AP is 19 and the 8th term is 22.find the sum of the first 40th terms of the series

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Question 1084676: the 7th term of an AP is 19 and the 8th term is 22.find the sum of the first 40th terms of the series
Answer by Boreal(15235) About Me  (Show Source):
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an=a1+(n-1)d
d=3, between 7th and 8th term
a7=a1+(n-1)d
a7=a1+6*3=a1+18
19=a1+18
a1=1
a40=1+(39)*3=118
sum=(40/2)(a1+a40)=20+(1+118)=119*20=2380