[tex](d): ax + by + c = 0 ,\ \ \ \ A(x_0,\ y_0).
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dist(A, d) = \dfrac{|ax_0+by_0+c|}{\sqrt{a^2+b^2}} [/tex]
[tex]\it (d): x-y-1=0,\ \ \ \ A(1,\ 2)
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dist(A,\ d)= \dfrac{|1-2-1|}{\sqrt{1+1}} = \dfrac{|-2|}{\sqrt2} = \dfrac{2}{\sqrt2} =\dfrac{\sqrt2\cdot\sqrt2}{\sqrt2} =\sqrt2[/tex]
