a6=a1 + 5d, a1+5d=0 ⇒a1= -5d.
S12=(2a1+11d)·12/2=(2a1+11d)·6, (2a1+11d)·6= -18⇒ 2a1+11d= -3
-10d+11d=-3, d= -3.
a6=a1 + 5d, a1+5d=0 ⇒a1= -5d.
S12=(2a1+11d)·12/2=(2a1+11d)·6, (2a1+11d)·6= -18⇒ 2a1+11d= -3
-10d+11d=-3, d= -3.