Answer:
50
Step-by-step explanation:
suppose 12 is a1 ;then 4 is a5
Find the common difference
an = a1 + ( n - 1 ) d
4 = 12 + ( 5 - 1 ) d
4 - 12 = 4d
-8 = 4d
-8 ÷ 4 = 4 ÷ 4d
-2 = d
common difference is -2
Solving for a1
4 = a1 + ( 7 - 1 ) -2
4 = a1 + ( 6 ) -2
4 = a1 -12
4 + 12 = a1
16 = a1
16 is a1
Solving for a5
an = 16 + ( n - 1 ) -2
an = 16 + ( -2n + 2 )
an = 16 - 2n + 2
an = -2n + 16 + 2
an = -2n + 18
a5 = -2( 5 ) + 18
a5 = -10 + 18
a5 = 8
Solving for the sum of five terms
a1 + an
sn = --------------- × n
2
16 + 8
s5 = --------------- × 5
2
24
s5 = --------------- × 5
2
s5 = 12 × 5
s5 = 60
