# How to find the basis functions of a polynomial solved for boundary conditions?

ss=Sum[a[i]*x^i,{i,0,10}];
eq1=ss/.x->0
eq2=ss/.x->1
sol=Solve[{eq1==0,eq2==0}];
sol1=ss/.sol


How can I find the coefficients of a[2],a[3],....,a[10] which would be (-x+x^2),(-x+x^3),...(-x+x^10).

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You should use Coefficient

s = Sum[a[i]*x^i, {i, 0, 10}]
(* a[0] + x a[1] + x^2 a[2] + x^3 a[3] + x^4 a[4] + x^5 a[5] +
x^6 a[6] + x^7 a[7] + x^8 a[8] + x^9 a[9] + x^10 a[10] *)

sol = Solve@{
0 == s /. x -> 0,
0 == s /. x -> 1
}
(* {{a[0] -> 0,
a[10] -> -a[1] - a[2] - a[3] - a[4] - a[5] - a[6] - a[7] - a[8] -
a[9]}} *)

pol = s /. First@sol
(* x a[1] + x^2 a[2] + x^3 a[3] + x^4 a[4] + x^5 a[5] +
x^6 a[6] + x^7 a[7] + x^8 a[8] +
x^10 (-a[1] - a[2] - a[3] - a[4] - a[5] - a[6] - a[7] - a[8] -
a[9]) + x^9 a[9] *)

Table[
Coefficient[pol, a[k], 1]
, {k, 0, 10}]
(* {0, x - x^10, x^2 - x^10, x^3 - x^10, x^4 - x^10, x^5 - x^10,
x^6 - x^10, x^7 - x^10, x^8 - x^10, x^9 - x^10, 0} *)