Question 944299
<pre>
Assume a quadratic general formula:

{{{a[n]=An^2+bn+c}}}, {{{a[1]=701495}}}, {{{a[2]=841853}}}, {{{a[3]=676836}}}

Substitute n=1

{{{a[1]=A(1)^2+B(1)+C}}}

{{{701495=A+B+C}}}

Substitute n=2

{{{a[2]=A(2)^2+B(2)+C}}}

{{{841853=4A+2B+C}}}

Substitute n=3

{{{a[3]=A(3)^2+B(3)+C}}}

{{{676836=9A+3B+C}}}

Now we have this system of equations to solve:

{{{system(A+B+C=701495,4A+2B+C=841853,9A+3B+C=676836)}}}

Solve that and get A = -152687.75, B=598420.5, C=255762

So the formula is {{{a[n]=-152687.75n^2+598420.5n+255762}}}

Substituting n=4 gives

{{{a[4]=-152687.75(4)^2+598420.5(4)+255762}}}

{{{a[4]=206444}}}

Edwin</pre>