Question

find 20th term of an AP whose 3rd and 8th tern are 5 and -15​

Answer 1

$\huge\red{Answer}$

Let a be the first term of this AP

and d be the common difference

a3 = 5

=> a+2d = 5 ----------------------------(1)

a8 = -15

=> a+7d = -15 ----------------------------(2)

Subtracting eq 1 from eq 2

a+7d=-15

a+2d=5

---------------

5d= -20

----------------

=> d = -4

a+2d = 5

=> a+2×(-4) = 5

=> a-8 = -5

=> a = -5+8

=> a = 3

a20 = a+19d

= 3+19×(-4)

= 3-76

= -73

HOPE IT HELPS YOU !!