So, d=- 1.

an = a 1+(n- 1)d = 4-n+ 1 = 5-n .

sn=na 1+n(n- 1)d/2=4n-( 1/2)(n^2-n)=-( 1/2)n^2+(9/2)n。

an+sn=-( 1/2)n^2+(7/2)n+5=- 10。

Finishing: n 2-7N-30 = 0.

Solution: n=-3 (truncation), n= 10.