Let a1, a2, a3, \dots be a sequence. If a_n = a_{n - 1} - a_{n - 2} for all n≥3, and a1 = 11 and a2 = 7, then find a6.
Repeatedly using the recurrence formula with n = 3, 4, 5, 6 gives respectively:
a3=a2−a1=−4a4=a3−a2=−11a5=a4−a3=−7a6=a5−a4=4
+613 
