Question 882303
First find the equation of the plane.
AB=(-1,6,1)
BC=(-1,-4,1)
{{{n=ABxBC=(matrix(3,3,i,j,k,-1,6,1,-1,-4,1))}}}
{{{n=i(6(1)-(-4)(1))+((1)(-1)-(1)(-1))j+((-1)(-4)-(-1)(6))k}}}
{{{n=10i+0j+10k}}}
{{{n*(matrix(1,3,x-3,y+5,z-1))=0}}}
{{{10(x-3)+0(y+5)+10(z-1)=0}}}
{{{10x-30+10z-10=0}}}
{{{10x+10z=40}}}
{{{highlight_green(x+z=4)}}}
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Now that you have the equation of the plane,
{{{L=abs(ax[0]+by[0]+cz[0]+d)/sqrt(a^2+b^2+c^2)}}}
where the plane is defined by {{{ax+by+cz+d=0}}} and the point is 
({{{x[0]}}},{{{y[0]}}},{{{z[0]}}})

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{{{a=1}}}
{{{b=0}}}
{{{c=1}}}
{{{d=-4}}}
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{{{L=abs(1(2)+1(3)-4)/sqrt(1^2+0^2+1^2)}}}
{{{L=abs(2+3-4)/sqrt(2)}}}
{{{L=1/sqrt(2)}}}
{{{highlight(L=sqrt(2)/2)}}}