Geometric Pythagorean Triples

Does there exist integers a , b , c 0 a,b,c\ne 0 satisfying a 2 + b 2 = c 2 { a }^{ 2 }+{ b }^{ 2 }={ c }^{ 2 } , and a , b , c a,b,c are in geometric progression ?

No Yes

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2 solutions

Wen Z
Nov 10, 2016

Suppose there does exist a pythagorean triple satisfying the property. Then let k k be the common ratio.

a 2 + k 2 a 2 = k 4 a 2 k 2 + 1 = k 4 a^2+k^2a^2=k^4a^2 \implies k^2+1=k^4 so k 2 k^2 is the golden ratio ( ϕ \phi ), and irrational which implies that k k is irrational (You can prove this by contrapositive). A similar approach can be taken if k < 1 k<1 in which k 2 k^2 must equal ϕ 1 \phi-1

Note that b = a k b=ak , c = a k 2 c=ak^2 , so a 2 + b 2 = c 2 a^2+b^2=c^2 would be substituted to a 2 + k 2 a 2 = k 4 a 2 a^2 + k^2 a^2 = k^4 a^2 , not what you have written. Just a typo. :)

Sharky Kesa - 4 years, 7 months ago

Yeah that's what I meant, I'll edit it.

Wen Z - 4 years, 7 months ago
Abhay Tiwari
May 12, 2016

The m , n m, n formula can come in handy here.

If we take two non-negative integers i.e. m n m \quad n

Then the Pythagorean triplets are m 2 n 2 , m 2 + n 2 a n d 2 m n {m^2-n^2}, {m^2+n^2} and {2mn}

And can be related to each other as ( m 2 n 2 ) 2 + ( 2 m n ) 2 = ( m 2 + n 2 ) 2 ({m^2-n^2})^2 + ({2mn})^2 = ({m^2+n^2})^2

Considering a = ( m 2 n 2 ) , b = ( 2 m n ) a n d c = ( m 2 + n 2 ) a=({m^2-n^2}), \quad b=({2mn}) and \quad c=({m^2+n^2}) Condition for geometric progression b 2 = a c b^2=ac , which will not be followed by the above mentioned value of a a , b b and c c . Thus, the given statement is not possible.

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