Integer side, prime area?

Geometry Level 3

Given a triangle, length of its three sides are all integers. It is possible that the numerical value of its area is a prime?

No Yes

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1 solution

Chan Tin Ping
Nov 21, 2017

Let the sides are a , b , c a, b, c , area is P P . There exist a theorem which shows the relationship between the sides of triangle and its area, which is ( s ) ( s a ) ( s b ) ( s c ) = P ( s = a + b + c 2 ) \sqrt{(s)(s-a)(s-b)(s-c)}=P \space \space(s=\frac{a+b+c}{2}) 1 4 ( a + b + c ) ( a + b c ) ( a b + c ) ( a + b + c ) = P \frac{1}{4}\sqrt{(a+b+c)(a+b-c)(a-b+c)(-a+b+c)}=P ( a + b + c ) ( a + b c ) ( a b + c ) ( a + b + c ) = 16 P 2 (a+b+c)(a+b-c)(a-b+c)(-a+b+c)=16P^2 Let a + b + c = w a + b c = x a b + c = y a + b + c = z \begin{aligned} a+b+c&=w\\ a+b-c&=x\\ a-b+c&=y\\ -a+b+c&=z \end{aligned} We can get w = x + y + z w=x+y+z , a = w z 2 a=\frac{w-z}{2} , b = w y 2 b=\frac{w-y}{2} , c = w x 2 c=\frac{w-x}{2} . Hence, w , x , y , z w,x,y,z must share same parity, because a , b , c a,b,c are integers. But w x y z = 16 P 2 wxyz=16P^2 , which means one of w , x , y , z w,x,y,z is even, so w , x , y , z w,x,y,z must be even numbers.

Let w = 2 w w=2w^{'} , x = 2 x x=2x^{'} ,. y = 2 y y=2y^{'} , z = 2 z z=2z^{'} . By ( w x y z = 16 P 2 wxyz=16P^2 ), and ( w = x + y + z w=x+y+z ), we can get ( P 2 = w x y z P^2=w^{'}x ^{'}y ^{'}z^{'} ) and ( w = x + y + z w^{'}=x ^{'}+y ^{'}+z^{'} )(obviously w w is the most biggest). Which means that we need to factorize P 2 P^2 into four integers, such that sum of the three factors equal to the biggest factor. Let P P is a prime number, P 2 P^2 can be express as ( 1 × 1 × P × P 1\times 1\times P\times P ) or ( 1 × 1 × 1 × P 2 1 \times 1 \times 1 \times P^2 ) only. Obviously, 1 + 1 + P > P 1+1+P>P , and 1 + 1 + 1 = 3 P 2 1+1+1=3\neq P^2 as 3 3 is not a perfect square.

Hence, there d o e s n o t \large does\space not exist triangle with integer side with prime area.

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