Solver To calculate surface area of a torus

Source code of 'To calculate surface area of a torus'

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==section input
For a Torus Inner radius *[input r=5] and Outer radius *[input R=6] are given.The surface area of is.

==section solution
perl
if($r<=0 || $R<=0)
{
print " Please enter the value of radius greater than 0 to calculate surface area of Torus";
}
else{
my $area=($R+$r)*($R-$r)*3.14159265*3.14159265;
my $b=($R-$r)/2;
my $a=($R+$r)/2;

print "
<A HREF=Geometry-Surface-Area-and-Volume.lesson> Geometry Surface Area and Volume of 3 Dimensional Figures</A>
Torus is of a doughnut shape.

The Formula for Surface area of Torus when inner and outer radius are given is
{{{Area=pi^2(Outer Radius-Inner Radius)*(Outer Radius+Inner Radius)}}}

{{{Area=pi^2*($R+$r)*($R-$r)=$area}}}

If the b is the radius of tube and a is the distance from the center of the tube to the center of the torus then,
{{{Area=4pi^2ab}}}

where {{{b=(Outer Radius-Inner Radius)/2}}}

{{{b=($R-$r)/2=$b}}}

and {{{a=(Outer Radius+Inner Radius)/2}}}

{{{a=($R+$r)/2=$a)}}}

Hence {{{Area=4*pi^2*$a*$b=$area}}}

also, {{{Outer Radius=a+b}}}
{{{Outer Radius=$a+$b=$R}}}

{{{Inner Radius=a-b}}}
{{{Inner Radius=$a-$b=$r}}}

Hence, The Surface Area of Torus of Inner radius $r and Outer radius $R is $area.

For more on this topic, See the lessons on <A HREF=http://www.algebra.com/algebra/homework/Surface-area/Lessons.html>Geometry: Area and Surface Area</A>

Some more is on <A HREF=http://www.algebra.com/algebra/homework/Surface-area/Surface-Area-and-Volume.lesson> Surface Area and Volume</A> and <A HREF=http://www.algebra.com/algebra/homework/Surface-area/Perimeter-and-Area.lesson> Perimeter and Area</A>.

";
}

==section output
area,

==section check
 R=5 r=2 area=207.261692