Ratio problem 2 by Dhaval Furia

Algebra Level pending

The product of two positive numbers is 616 616 . If the ratio of the difference of their cubes to the cube of their difference is 157 : 3 157:3 , what is the sum of the two numbers?

58 58 85 85 50 50 95 95

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2 solutions

Chew-Seong Cheong
Jun 14, 2020

Let the two positive numbers be a a and b b , such that a > b a>b . Then a b = 616 ab =616 and

a 3 b 3 ( a b ) 3 = 157 3 3 ( a 3 b 3 ) = 157 ( a b ) 3 3 ( a b ) ( a 2 + a b + b 2 ) = 157 ( a b ) 3 3 ( a 2 + a b + b 2 ) = 157 ( a 2 2 a b + b 2 ) 3 ( a 2 + 2 a b + b 2 ) 3 a b = 157 ( a 2 + 2 a b + b 2 ) 628 a b 3 ( a + b ) 2 3 a b = 157 ( a + b ) 2 628 a b 154 ( a + b ) 2 = 625 a b ( a + b ) 2 = 625 616 154 = 2500 a + b = 50 \begin{aligned} \frac{a^3-b^3}{(a-b)^3} & = \frac {157}3 \\ 3(a^3-b^3) & = 157(a-b)^3 \\ 3(a-b)(a^2+ab+b^2) & = 157(a-b)^3 \\ 3(a^2+ab+b^2) & = 157(a^2-2ab+b^2) \\ 3(a^2+2ab+b^2) - 3ab & = 157(a^2+2ab+b^2) - 628ab \\ 3(a+b)^2 - 3ab & = 157(a+b)^2 - 628ab \\ 154(a+b)^2 & = 625ab \\ (a+b)^2 & = \frac {625 \cdot 616}{154} = 2500 \\ \implies a+b & = \boxed{50} \end{aligned}

Let the numbers be a a and b b . Then a b = 616 ab=616

a 3 b 3 ( a b ) 3 = 157 3 \dfrac{a^3-b^3}{(a-b) ^3}=\dfrac{157}{3}

Solving, we get a 2 + b 2 = 1268 ( a + b ) 2 = a 2 + b 2 + 2 a b = 1268 + 1232 = 2500 a + b = 50 a^2+b^2=1268\implies (a+b)^2=a^2+b^2+2ab=1268+1232=2500\implies a+b=\boxed {50} .

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