A triangle with sides $3a-1$, $a^2 + 1$ and $a^2 + 2$ has a perimeter of 16 units. What is the number of square units in the area of the triangle?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
A triangle with sides $3a-1$, $a^2 + 1$ and $a^2 + 2$ has a perimeter of 16 units. What is the number of square units in the area of the triangle?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
12\text{ square units}
A triangle with sides $3a-1$, $a^2 + 1$ and $a^2 + 2$ has a perimeter of 16 units. What is the number of square units in the area of the triangle?
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