Question 1053537
{{{a[6]=a[1]+(6-1)*d=29}}}
{{{a[1]+5d=29}}}
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{{{a[13]=a[1]+(13-1)*d=64}}}
{{{a[1]+12d=64}}}
Subtracting the previous equation from this one,
{{{a[1]+12d-a[1]-5d=64-29}}}
{{{7d=35}}}
{{{d=5}}}
Then,
{{{a[1]+5(5)=29}}}
{{{a[1]+25=29}}}
{{{a[1]=4}}}
So then,
{{{S[20]=(n/2)(a[1]+a[20])=(20/2)(4+4+19*5)=10*103=1030}}}