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a1=2
r=7-2=5
Sn=1/2*(2a1+r(n-1))*n
1/2*(2*2+5(n-1))*n=245 /*2
4n+5n²-5n=490
5n²-n-490=0
Δ=1+4*5*490=9801
√Δ=99
n∈N
n=1/10*(1+99)=10
x=a10=2+5*9
x=2+45
x=47