please answer this question

Answers
Step-by-step explanation:
S9 = 81
S20 = 400
first term a=?
common difference d =?
sum of first 9 terms
S9 = 9/2[2a+(9-1)d]
81 = 9/2[2a+8d]
81/9 = (a+4d)
a+4d = 9....................1
sum of first 20 terms
S20 = 20/2[2a+(20-1)d]
400 = 10[ 2a+19d]
400/10 = 2a+19d
......................2
by elimination method
a+4d = 9. ×2
2a + 8d= 18
2a+19d = 40
-. - -
__________
0 - 11d = -22
d = -22/-11
d = 2
put value of d in equation 1
a+4d= 9
a +4×2=9
a+8 = 9
a = 9-8
a = 1
Step-by-step explanation:
Question:-
The sum of first 9 terms of an AP is 81 and the sum of its first 20 terms is 400. Find the first term and the common difference of the AP.
Solution:-
As, we know that :-
The formula for finding the Sum of n terms is:-
Sum of 9 terms = 81
Sum of 20 terms = 400