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13-06-2017
Mathematics

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What is the 21st term of an arithmetic sequence with terms a6=50 and a11=85

Respuesta :

16-06-2017

a1+d(n-1)
a1+d(6-1)=50
a1+5d=50
a1+d(11-1)=85
a1+10d=85
a1=50-5d
(50-5d)+10d=85
50-5d+10d=85
5d+50=85
5d=35
d=7
a1+7(6-1)=50
a1+35=50
a1=15
15+7(n-1)
15+7(21-1)
15+7(20)
15+140=155

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