Question

(9+99+999+.........+n terms)

Answer 1

Answer:

a=9 d=90

sn=n/2(18+90n-90)

n/2(90n-72)

45n^2-36n

Answer 2

Step-by-step explanation:

we can split this sequence into (10+10raise to 2+10raise to3.....)-(1+1+1+.....+n terms) see their sum separately and add