aditya.khare
Bunuel
A sequence \(a_n\) defined such that \(a_{n} = a_{n-1} - a_{n-2}\), where n is a positive integer greater than 2. What is the value of \(a_5\)? \(a_{3} = a_{2} - a_{1}\); \(a_{4} = a_{3} - a_{2} = (a_{2} - a_{1}) - a_{2} = -a_{1}\); \(a_{5} = a_{4} - a_{3} = (-a_{1}) - (a_{2} - a_{1}) = -a_{2}\); \(a_{6} = a_{5} - a_{4} = (-a_{2}) - (-a_{1})=-(a_{2} - a_{1}) = -a_{3}\); \(a_{7} = a_{6} - a_{5} = -(a_{2} - a_{1}) - (-a_{2})=a_{1} = -a_{4}\); \(a_{8} = a_{7} - a_{6} = a_{1} - (a_{1} - a_{2})=a_{2} = -a_{5}\); and so on.
(1) \(a_{4} - a_{3} = 3\) Above we we got that \(a_{5} = a_{4} - a_{3}\), hence \(a_{5} = 3\). Sufficient. (2) \(a_{14} = -3\) \(a_{14} = -a_{11}=a_8=-a_5=-3\). Hence \(a_{5} = 3\). Sufficient. Answer: D.
So answer is D when a14 = -3. What if a14 = 1. In that case, from both statements we are getting an answer but a different one. So will the question becomes invalid or we answer A or D?
Remember, on the GMAT, two data sufficiency statements always provide true information, and these statements never contradict each other or the stem. If the statements contradict each other, then, as per GMAT standards, the question is flawed. Hence, for the options not to contradict, \(a_{14}\) must be -3 in statement (2). Alternatively, if \(a_{14} = 1\), then in statement (1), \(a_{4} - a_{3}\) must be -1. In either case, though, each statement is sufficient on its own.