(1)
an=a1+(n-1)d
a2+a4=-6
2a1+4d=-6
a1+2d=-3 (1)
a3+a5=-2
2a1+6d=-2
a1+3d=-1 (2)
(2)-(1)
d=2
a1= -7
an = -7+(n-1)2 = 2n-9
(2)
an >0
2n-9 >0
n> 9/2
|an| = -an ; n=1,2,3,4
= an ; n>=5
Tn = |a1|+|a2|+...+|an|
= -(a1+a2+a3+a4) +(a5+a6+...+an)
= (7+1)2 + ( 2n-8)(n-4)/2
= 16 + (n-4)^2
=n^2-8n + 32