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lisabon 2012 [21]
3 years ago
7

1. The sum of first n terms of an arithmetic sequence is 3n²+4n

Mathematics
1 answer:
valentina_108 [34]3 years ago
5 0

Answer:

<h3>           a.  a₁=7,     b.  a_n=7n+1,    c.  a₂₀=141</h3>

Step-by-step explanation:

a.

 a_1=S_1=3\cdot1^2+4\cdot1=3+4=7

b.

   a_1+a_2=S_2\\\\a_1+a_1+d=3\cdot2^2+4\cdot2\\\\7+7+d=3\cdot4+8\\\\14+d=12+8\\\\d=6\\\\\\a_n=a_1+d(n-1)\\\\a_n=7+6(n-1)\\\\a_n=7+6n-6\\\\\underline{a_n=7n+1}

c.

 a_{20}=7\cdot20+1=141

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