 All about flooble | fun stuff | Get a free chatterbox | Free JavaScript | Avatars  perplexus dot info  Diophantine And Almost Fermat (Posted on 2007-02-21) Consider three positive integers x< y< z in arithmetic sequence.

Determine analytically all possible solutions of each of the following equations:

(I) x3 + z3 = y3 + 10yz

(II) x3 + y3 = z3 - 2, with y< 116.

 Submitted by K Sengupta Rating: 4.0000 (1 votes) Solution: (Hide) Solution of (I):Since x, y and z are positive integers in arithmetic sequence with z>y>x, we can substitute z = y+a and x = y-a where a is a positive integer. So, x^3+z^3 = y^3 + 10yz Or, 2y(y^2 + 3*a^2) = y^3 + 10y(y+a) Or, y^2 + 6*a^2 = 10(y+a)( since y is positive, y=0 ia a contradiction) Or, 6*A^2 + B^2 = 175, where (A,B) = (y-5, 6a-5) Or, (A,B) = (1,13),(-1,13), (1,-13), (-1,-13), (3,11), (-3,11), (3,-11), (-3,-11), (5,5), (-5,5), (5,-5),(-5,-5) Of the abovementioned choices of (A,B), only (A,B) = (1,13), (-1,13) yields positive integer values for y and a. Now, (y-5, 6a-5) = (1,13); (-1,13) gives: (y,a) =(6,3), (4,3), so that: (x,y,z) = (3,6,9), (1,4,7) constitutes all possibl;e solutions to the given problem. Solution of (II):Let x = y-a and z = y+a Then,x^3 + y^3 = z^3 - 2 yields: y^2(y-6a) = 2(a^3-1)...(i) Since, LHS must be divisible by 2, it follows that y is even. So substituting y = 2p, we obtain: 4*p^2(p-3a)+ 1 = a^3 Or, a^3 = 1 (Mod 4) Hence, y is even and a is odd....(ii) Now, if y<6a then, LHS of (i) is negative, while RHS of (i) is non-negative. This is a contradiction. Hence, y>=6a. Case A: y = 6a From (i), we obtain: a^3 = 1, giving a=1, so that y=6, yielding: (x,y,z) = (5,6,7) Case B: y is greater than 6a Minimum value of y is (6a+1), so that: (6a+1)^2 <= y^2(y-6a)= 2(a^3-1) So, 2*a^3 >= 36*a^2 + 12a +3 > 36*a^2 Or, a^3> 18*a^2, giving: a>18, so that a>=19, yielding: y> 6a+1>= 115, so that y>=116 as y must be even Consequently, (x,y,z) = (5,6,7) is the only possible solution whenever y<116. Comments: ( You must be logged in to post comments.)
 Subject Author Date Solution - Equation I hoodat 2007-02-28 13:33:54 Separate equations Gamer 2007-02-27 01:56:39 possible solutions hoodat 2007-02-26 18:20:24 Hint K Sengupta 2007-02-22 13:51:46 Please log in:
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