First differences of the y-values for the consecutive x-values are ...
-3 -(-4.5) = 1.5
-1.5 -(-3) = 1.5
0 -(-1.5) = 1.5
1.5 -0 = 1.5
The first differences are constant, so the function is linear.
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Since we know the function is linear, there's no real point in computing the ratios of successive outputs. In any event, we know it is not constant. The ratio with 0 as a numerator will be 0; the ratio with 0 as a denominator will be undefined.
