127) what is a1

[ern5231]

an=an-1-2an-2, n>=3. Given that a4=-12, a3=0. What is the value of a1? (note : subscripts are provided in bold)

a)3
b)4
c)5
d)6
e)8

[nicktimoshenko]

in first find a2

a4 = a3 - 2*a2

so a2 = (a3-a4)/2

after that find a1

[arohan_sambyal]

answer : 3
given a4=-12 and a3=0
a4=a3-2a2=>-12=0-2a2=>a2=6
a3=a2-2a1=>0=6-2a1=>6=2a1=>a1=3