f(x) = ln(x)
f'(x) = 1/x
f(1) = 0
f'(1) = 1
Now we know the slope of the line, let's search for the costant term:
1*x+a=f(x), for x=1:
1*1+a=0 => a=-1
linear approx: L(x) =1*x-1.
L(1.38) = 1*1.38-1 = 0.38 ~= ln(1.38)
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