1) Escrever a P.A cujo terceiro termo é 7 e o décimo é 21
a10=a1+9r
a3=a1+2r
21=a1+9r
7=a1+2r
21/7=9r-2r
7r=14
r=41/7
r=2
7=a1+4
7-4=a1
a1=3
(3,5,7,9,11,13,15,17,19,21)
3,5,7,9,11,13,15,17,19,21
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Verified answer
a10=a1+9r
a3=a1+2r
21=a1+9r
7=a1+2r
21/7=9r-2r
7r=14
r=41/7
r=2
7=a1+2r
7=a1+4
7-4=a1
a1=3
(3,5,7,9,11,13,15,17,19,21)
3,5,7,9,11,13,15,17,19,21