The sum of first 7 terms of an arithmetic sequence 280 and sum of first 12 terms is 680.
Find its 4th term
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Answer:
4th term is -40
Step-by-step explanation:
S₇ = a +(7-1)d
280 = a + 6d
S₁₂= a + 11d
680 = a +11d
a= 608 - 11d
subsstitute value of a in 1st equation
280 = 680 - 11d +6d
5d = 400
d= 80
put d= 80 in S₇ equation
280= a + 480
a= -200
Now for 4th term
t₄= a + (4-1) d
t₄ = -200 + 3*80
t₄ = -200 + 240
t₄ = -40
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