Question 997593
6-1.8+0.54-0.162+...
<pre>
{{{matrix(1,2,2nd,term)/matrix(1,2,1st,term)=(-1.8)/6=-0.3}}}

{{{matrix(1,2,3rd,term)/matrix(1,2,2nd,term)=0.54/(-1.8)=-0.3}}}

{{{matrix(1,2,4th,term)/matrix(1,2,3rd,term)=(-0.162)/(0.54)=-0.3}}}

So there is a common ratio of -0.3.  This is a geometric 
series with r=-0.3 and first term a<sub>1</sub>=6.

Substitute those with n=8 in the sum formula:

{{{S[n]=(a[1](1-r^n))/(1-r)}}}

{{{S[8]=(6(1-(-0.3)^8))/(1-(-0.3))}}}

{{{S[8]=(6(1-(-0.3)^8))/(1+0.3)}}}

{{{S[8]=(6(1-(-0.3)^8))/1.3}}}

{{{S[8]=4.6150818}}}

Edwin</pre>