Algebra or Geometry

Let a , b , c , p , q , r > 0 a, b, c, p, q, r > 0 such that ( a , b , c ) (a,b,c) is a geometric progression and ( p , q , r ) (p, q, r) is an arithmetic progression. If a p b q c r = 6 and a q b r c p = 29 a^p b^q c^r = 6 \quad \text{and} \quad a^q b^r c^p = 29 then compute a r b p c q \lfloor a^r b^p c^q \rfloor .


The answer is 29.

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2 solutions

Sharky Kesa
Aug 21, 2017

This was a nice troll question. Let b = a r b=ar , c = a r 2 c=ar^2 , q = p + d q=p+d and r = p + 2 d r=p+2d . Thus, the second statement says a p + d ( a r ) p + 2 d ( a r 2 ) p = a 3 p + 3 d r 3 p + 2 d = 29. a^{p+d}(ar)^{p+2d}(ar^2)^p = a^{3p+3d} r^{3p+2d} = 29.

Note that a r b p c q = a p + 2 d ( a r ) p ( a r 2 ) p + d = a 3 p + 3 d r 3 p + 2 d = 29 a^r b^p c^q = a^{p+2d} (ar)^{p} (ar^2)^{p+d} = a^{3p+3d} r^{3p+2d} = 29 from above. Thus, the answer is 29 \boxed{29} .

L e t a = a , . . . . . b = a x , . . . . . c = a x 2 . . . . . . . . . . . . . . . . . . . . . . . p = p , . . . . . . . . . . q = p + d , . . . . . . . . . . r = p + 2 d . S o a p b q c r = a p ( a x ) p + d ( a x 2 ) p + 2 d = a 3 p a 3 d x 3 p x 5 d = 6. A l s o a q b r c p = a p + d ( a x ) p + 2 d ( a x 2 ) p = a 3 p a 3 d x 3 p x 2 d = 29 W e w a n t a r b p c q = a p + 2 d ( a x ) p ( a x 2 ) p + d = a 3 p a 3 d x 3 p x 2 d = 29. Let ~~a=a,.....b=a*x,.....c=a*x^2.......................p=p,..........q=p+d,..........r=p+2d.\\ So~~a^p*b^q*c^r=a^p*(a*x)^{p+d}*(a*x^2)^{p+2d}=a^{3p}*a^{3d}~*~x^{3p}*x^{5d}=6.\\ Also~~a^q*b^r*c^p=a^{p+d}*(a*x)^{p+2d}*(a*x^2)^{p}=\color{#20A900}{a^{3p}*a^{3d}~*~x^{3p}*x^{2d}=29}\\ We~want~~a^r*b^p*c^q=a^{p+2d}*(a*x)^{p}*(a*x^2)^{p+d}=\color{#20A900}{a^{3p}*a^{3d}~*~x^{3p}*x^{2d}=\Large \color{#D61F06}{29}.}

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