That is against the rules!

Algebra Level 2

log ( a + b + c ) = log ( a ) + log ( b ) + log ( c ) \large \log(a+b+c) = \log(a)+\log(b)+\log(c)

a < b < c a<b<c are positive integers that satisfy the equation above. Find a 2 b 3 c 4 a^2b^3c^4 .


The answer is 648.

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2 solutions

Pop Wong
Sep 6, 2020

Given that a , b a, b and c c are positive integers and a < b < c a<b<c ,

log ( a + b + c ) = log ( a ) + log ( b ) + log ( c ) log ( a + b + c ) = log ( a b c ) a + b + c = a b c suppose a 2 , then b 3 a + b + c = a b c b + c = a ( b c 1 ) min. of RHS = 2 ( 3 c 1 ) = 6 c 2 = c + ( 5 c 2 ) > LHS as b < 5 c 2 for c 3 contradiction a = 1 1 + b + c = b c c ( b 1 ) = b + 1 c = b + 1 b 1 b = 2 1 + 2 + c = 2 c c = 3 a 2 b 3 c 4 = 1 2 2 3 3 4 = 1 8 81 = 648 \begin{aligned} \log(a+b+c) &= \log(a) + \log(b) + \log (c) \\ \log(a+b+c) &= \log(a\cdot b \cdot c) \\ \implies a+b+c &= a \cdot b \cdot c \\ \text{ suppose } a \geq 2, \text{ then } b\geq 3 \\ a+b+c &= a\cdot b \cdot c \\ \implies b+c &= a(bc - 1) \\ \text{min. of RHS } &= 2 ( 3c - 1) = 6c - 2 = c + (5c -2 ) \text{ > LHS as } b < 5c-2 \text{ for c } \geq 3 \implies \text{ contradiction } \implies \boxed{a = 1} \\ 1+b+c &= b\cdot c \\ \implies c(b-1) &= b+ 1 \implies c = \cfrac{b+1}{b-1} \implies \boxed{b = 2 } \implies 1+2+c=2c \implies \boxed{c = 3 }\\ \\ a^2 b^3 c^4 &= 1^2 2^3 3^4= 1\cdot 8 \cdot 81 = \boxed{648} \end{aligned}

A perfect number isn't that whose factors multiply to the number, only the sum matters, 28 is also perfect number, but 1 2 4 7 14 28 1 \cdot 2 \cdot 4 \cdot 7 \cdot 14\neq 28

Vinayak Srivastava - 9 months, 1 week ago

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Oops... thanks.

Pop Wong - 9 months, 1 week ago

log ( a + b + c ) = log a + log b + log c log ( a + b + c ) = log ( a b c ) a + b + c = a b c \begin{aligned} \log(a+b+c) & = \log a + \log b + \log c \\ \log(a+b+c) & = \log(abc) \\ \implies a + b + c & = abc \end{aligned}

This implies that a = 1 a=1 , b = 2 b=2 , and c = 3 c=3 , and a 2 b 3 c 4 = 1 × 8 × 81 = 648 a^2b^3c^4 = 1 \times 8 \times 81 = \boxed{648} .

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