Question 357911
{{{a[n]=a[1]+(n-1)*d}}}
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{{{d=a[2]-a[1]=4-2=2}}}
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{{{n-1=(a[n]-a[1])/d}}}
{{{n-1=(120-2)/2=59}}}
{{{n=60}}}
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{{{S[60]=(60(2+120))/2}}}
{{{highlight(S[60]=3660)}}}