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r=4
x=an
an=a1+(n-1)*r
an=2+(n-1)*4
an=2+4n-4=4n-2
Sn=242=0,5(a1+an)*n=0,5(2+4n-2)*n=0,5*4n²=2n²
242=2n²
242 -2n²=0/:2
121-n²=0
(11-n)*(11+n)=0
n=11 ∨n=-11∉N+
n=11
a11= a1+(11-1)*r=2+10*4=42
x=an=42