How many right triangles, whose sides are all positive whole numbers, have the property that the area is numerically equal to perimeter?

1 Answer
Jul 28, 2018

Let a,band c a,bandc are three sides of a right triangle where cc is the hypotenuse , This means c>a and c>bc>aandc>b.

As per given condition of the problem a,band c a,bandc are all positive whole numbers ,
and

1/2ab =a+b+c.....[1]

Again by Pythagorean theorem

c^2=a^2+b^2........[2]

Combining [1] and [2] we get

=>(1/2ab-a-b)^2=a^2+b^2

=>1/4a^2b^2+a^2+b^2-2*1/2a^2b-2*1/2ab^2+2ab=a^2+b^2

=>1/4a^2b^2-a^2b-ab^2+2ab=0

=>1/4ab-a-b+2=0. as ab!=0

=>1/2ab-2a-2b+4=0........[3]

Combining {1} and {3] we get

2a+2b -4=a+b+c.

=>c=a+b-4......[4]

Combining [1] and [4] we get

2a+2b-4=1/2ab

=>a+b=2+1/4ab......[5]

LHS of this relation is an integer. To satisfy this both a and b should be even or any of aand b is a multiple of 4. So minimum value of a or b=4. If minimum value of b=4 then minimum value of a will be 3 and c=5 But it does not satisfy equation [1]
For integer values of a,band c satisfying these conditions and equation {2] we get the following when b=8,the next integral multiple of 4

a=6,b=8and c=10

For b taking other higher multiple of 4 as 12,16,20..etc the relation (5) is not satisfied.