a)a6=20 a6=a1+5r => a1=20-5r
a10=4 a10=a1+9r
20-5r+9r=4
4r=-16
r=-4
a1=40
an=40-4(n-1)
b)a3=9 a3=a1+2r => a1=9-2r
a15=-3 a15=a1+14r
9-2r+14r=-3
12r=-12
r=-1
a1=11
an=11-(n-1)
c)a3=4 a3=a1+2r => a1=4-2r
a5=6=a1+4r
4-2r+4r=6
2r=2
r=1
a1=-2
an=2+1(n-1)
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a)a6=20 a6=a1+5r => a1=20-5r
a10=4 a10=a1+9r
20-5r+9r=4
4r=-16
r=-4
a1=40
an=40-4(n-1)
b)a3=9 a3=a1+2r => a1=9-2r
a15=-3 a15=a1+14r
9-2r+14r=-3
12r=-12
r=-1
a1=11
an=11-(n-1)
c)a3=4 a3=a1+2r => a1=4-2r
a5=6=a1+4r
4-2r+4r=6
2r=2
r=1
a1=-2
an=2+1(n-1)