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2 years ago

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

Mathematics

1 answer:

valentina_108 [34]2 years ago

5 0

Answer:

Step-by-step explanation:

$a_1=S_1=3\cdot1^2+4\cdot1=3+4=7$

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

$a_{20}=7\cdot20+1=141$

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