Part 1

Algebra Level 5

{ a 2 + b 2 + c 2 + d 2 + e 2 = a ( b + c + d + e ) 2011 a + 2012 b + 4026 c + 2014 d + 4030 e = 16232832 \large \left\{\begin{matrix} a^2+b^2+c^2+d^2+e^2=a(b+c+d+e) & & \\ 2011a+2012b+4026c+2014d+4030e=16232832 & & \end{matrix}\right. Find the value of below expression. a + b + c + d + e 9 \frac{a+b+c+d+e}{9}


The answer is 672.

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2 solutions

Trần Thế Huy
Jan 15, 2016

Tom Engelsman
Sep 6, 2017

The main trick is to multiply the first equation through by 4 which results in:

4 ( a 2 + b 2 + c 2 + d 2 + e 2 ) = 4 a ( b + c + d + e ) ; 4(a^2 + b^2 + c^2 + d^2 + e^2) = 4a(b + c + d + e);

or ( a 2 4 a b + 4 b 2 ) + ( a 2 4 a c + 4 c 2 ) + ( a 2 4 a d + 4 d 2 ) + ( a 2 4 a e + 4 e 2 ) = 0 ; (a^2 - 4ab + 4b^2) + (a^2 - 4ac + 4c^2) + (a^2 - 4ad + 4d^2) + (a^2 - 4ae + 4e^2) = 0;

or ( a 2 b ) 2 + ( a 2 c ) 2 + ( a 2 d ) 2 + ( a 2 e ) 2 = 0 ; (a - 2b)^2 + (a - 2c)^2 + (a - 2d)^2 + (a - 2e)^2 = 0;

or b = c = d = e = a 2 . b = c = d = e = \frac{a}{2}.

If we now substitute these values into the second linear equation, we now obtain:

2011 a + ( a 2 ) ( 2012 + 4026 + 2014 + 4030 ) = 16232832 a = 2016 ; 2011a + (\frac{a}{2})(2012 + 4026 + 2014 + 4030) = 16232832 \Rightarrow a = 2016;

and our final calculation comes to:

a + b + c + d + e 9 = a + 4 ( a 2 ) 9 = a 3 = 2016 3 = 672 . \frac{a + b + c + d + e}{9} = \frac{a + 4(\frac{a}{2})}{9} = \frac{a}{3} = \frac{2016}{3} = \boxed{672}.

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