Answer:
10
Step-by-step explanation:
f ( 1 ) = 18
First term ( a ) = 18
f ( n + 1 ) = f ( n ) - 2
When, n = 1
f ( 1 + 1 ) = f ( 1 ) - 2
f ( 2 ) = 18 - 2
f ( 2 ) = 16
f ( 2 ) - f ( 1 )
= 16 - 18
= - 2
Common difference ( d ) = - 2
f ( 5 )
= a + 4d
= 18 + 4 ( - 2 )
= 18 - 8
= 10
