Question 969332
{{{a[20]=a[1]+20d=45}}}
{{{a[10]=a[1]+10d=25}}}
Subtract the second equation from the first,
{{{a[1]+20d-a[1]-10d=45-25}}}
{{{10d=20}}}
{{{d=2}}}
Then,
{{{a[1]+10(2)=25}}}
{{{a[1]+20=25}}}
{{{a[1]=25-20}}}
{{{a[1]=5}}}
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Look at the ratio of successive terms,
{{{-5/-10=1/2}}}
{{{-5/2/-5=1/2}}}
So each subsequent term is the previous term divided by 2.
{{{-10/2=-5}}}
{{{-5/2}}}
{{{-5/4}}}
{{{-5/8}}}
{{{-5/16}}}
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{{{sum(-10/(2^n))=sum(-10*(1/2)^n)=-10*sum((1/2)^n)=-10(2)=-20}}}