



 
Hi Lucy, $a$ certainly does not need to be negative. For example in the exponential function $f(x) = 5^x, a = 5.$ If $a$ is negative you need to take some extra care however. If $a = 3$ then the function $f(x) = a^x$ has the value at $x = 2$ of $f(2) = a^2 = (3)^2 = 9$ while $3^2 =  (3)^2 = 9.$ Penny  


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