just work backwards ...
20,22,24,26,28
The fifth term of an AP is 20 and the common different is -2.Find the first four terms
2 answers
In AP:
an = a1 + ( n - 1 ) d
In this case:
a5 = 20 , d = - 2
a5 = a1 + 4 d
20 = a1 + 4 ∙ ( - 2 )
20 = a1 - 8
Add 8 to both sides
28 = a1
a1 = 28
a2 = a1 + d = 28 + ( - 2 ) = 28 - 2 = 26
a3 = a1 + 2 d = 28 + 2 ∙ ( - 2 ) = 28 - 4 = 24
a4 = a1 + 3 d = 28 + 3 ∙ ( - 2 ) = 28 - 6 = 22
First four terms:
28 , 26 , 24 , 22
an = a1 + ( n - 1 ) d
In this case:
a5 = 20 , d = - 2
a5 = a1 + 4 d
20 = a1 + 4 ∙ ( - 2 )
20 = a1 - 8
Add 8 to both sides
28 = a1
a1 = 28
a2 = a1 + d = 28 + ( - 2 ) = 28 - 2 = 26
a3 = a1 + 2 d = 28 + 2 ∙ ( - 2 ) = 28 - 4 = 24
a4 = a1 + 3 d = 28 + 3 ∙ ( - 2 ) = 28 - 6 = 22
First four terms:
28 , 26 , 24 , 22