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单词 ProofOfPythagoreanTriples
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proof of Pythagorean triples


If a,b, and c are positive integers such that

a2+b2=c2(1)

then (a,b,c) is a Pythagorean tripleMathworldPlanetmath. If a,b, and c arerelatively prime in pairs then (a,b,c) is a primitivePythagorean triple. Clearly, if k divides any two of a,b,and c it divides all three. And if a2+b2=c2 thenk2a2+k2b2=k2c2. That is, for a positive integer k, if(a,b,c) is a Pythagorean triple then so is (ka,kb,kc).Hence, to find all Pythagorean triples, it’s sufficient to findall primitive Pythagorean triples.

Let a,b, and c be relatively prime positive integers suchthat a2+b2=c2. Set

mn=a+cb

reduced tolowest terms, That is, gcd(m,n)=1. From the triangle inequalitym>n. Then

mnb-a=c.(2)

Squaring both sides of (2) and multiplying through by n2we get

m2b2-2mnab+n2a2=n2a2+n2b2;

which, after cancelling and rearranging terms, becomes

b(m2-n2)=a(2mn).(3)

There are two cases, either m and n are of opposite parity, orthey or both odd. Since gcd(m,n)=1, they can not both beeven.

Case 1. m and n of opposite parity, i.e., m±n(mod  2). So 2 divides b sincem2-n2 is odd. From equation (2), n divides b.Since gcd(m,n)=1 then gcd(m,m2-n2)=1, therefore m alsodivides b. And since gcd(a,b)=1, b divides 2mn. Thereforeb=2mn. Then

a=m2-n2,b=2mn,and from(2),c=mn 2mn-(m2-n2)=m2+n2.(4)

Case 2. m and n both odd, i.e., m±n(mod  2). So 2 divides m2-n2. Then by the same processas in the first case we have

a=m2-n22,b=mn,andc=m2+n22.(5)

The parametric equations in (4) and (5) appearto be different but they generate the same solutions. To showthis, let

u=m+n2 and v=m-n2.

Then m=u+v, and n=u-v. Substituting those values for m andn into (5) we get

a=2uv,b=u2-v2,andc=u2+v2(6)

where u>v, gcd(u,v)=1, and u and v are of oppositeparity. Therefore (6), with a and binterchanged, is identical to (4). Thus since(m2-n2,2mn,m2+n2), as in (4), is aprimitive Pythagorean triple, we can say that (a,b,c) is aprimitive pythagorean triple if and only if there existsrelatively prime, positive integers m and n, m>n, such thata=m2-n2,b=2mn, and c=m2+n2 .

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更新时间:2025/5/4 11:30:55