Given a triangle, length of its three sides are all integers. It is possible that the numerical value of its area is a prime?
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Let the sides are a , b , c , area is 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 = 2 a + b + c ) 4 1 ( 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 ) = 1 6 P 2 Let a + b + c a + b − c a − b + c − a + b + c = w = x = y = z We can get w = x + y + z , a = 2 w − z , b = 2 w − y , c = 2 w − x . Hence, w , x , y , z must share same parity, because a , b , c are integers. But w x y z = 1 6 P 2 , which means one of w , x , y , z is even, so w , x , y , z must be even numbers.
Let w = 2 w ′ , x = 2 x ′ ,. y = 2 y ′ , z = 2 z ′ . By ( w x y z = 1 6 P 2 ), and ( w = x + y + z ), we can get ( P 2 = w ′ x ′ y ′ z ′ ) and ( w ′ = x ′ + y ′ + z ′ )(obviously w is the most biggest). Which means that we need to factorize P 2 into four integers, such that sum of the three factors equal to the biggest factor. Let P is a prime number, P 2 can be express as ( 1 × 1 × P × P ) or ( 1 × 1 × 1 × P 2 ) only. Obviously, 1 + 1 + P > P , and 1 + 1 + 1 = 3 = P 2 as 3 is not a perfect square.
Hence, there d o e s n o t exist triangle with integer side with prime area.