Tenth term of an AP is 21 and the sum of its first 10 terms is 120. Find the 20th term of AP.
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Step-by-step explanation:
a10=21
a+9d=21 equ. 1
S10=120
Sn= n/2[2a+(n-1)d]
n=10
sn=120
120=10/2[2a+(10-1)d]
120=5[2a+9d]
24=2a+9d. equ 2
multiple equ 1 by 2
= 2a+18d=42. equ. 3
equ 2 and equ 3
2a+18d=42.
2a+9d=24 substrate
=9d=18
d=2
a+9d=21
a+9×2=21
a+18=21
a=4
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