You Forgot Five!

Algebra Level 2

a + b + c + d = 1 b + c + d + e = 2 c + d + e + a = 3 d + e + a + b = 4 e + a + b + c = 6 \begin{aligned} a+b+c+d &=& 1 \\ b+c+d+e &=& 2 \\ c+d+e+a &=& 3 \\ d+e+a+b &=& 4 \\ e+a+b+c &=& 6\\ \end{aligned}

What is the value of a e + b d + c a^e + b^d + c ?


The answer is 9.

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3 solutions

Dick He
Feb 8, 2014

To find e:
Add all equations up to get 4a+4b+4c+4d+4e=16
Simplify: a+b+c+d+e=4
Subtract:
a+b+c+d+e=4


a+b+c+d=1

e=3

To find d with e:
first equation-fifth equation:
a+b+c+d=1

a+b+c=3

d=-2

To find a with d and e:
fifth equation-second equation:
a+b+c=3

b+c=1

a=2

To find b with a, d, and e:
Use fourth equation
-2+3+2+b=4
b=1

To find c with a, b, d, and e
Use fifth equation
3+2+1=6
c=0

Oliver Welsh
Feb 4, 2014

Adding all 5 5 sums gives 4 ( a + b + c + d + e ) = 16 a + b + c + d + e = 4 ( 1 ) 4*(a+b+c+d+e) = 16 \Rightarrow a+b+c+d+e=4 \quad (1) From this we can subtract the sums from ( 1 ) (1) (for example we would say ( a + b + c + d + e ) ( a + b + c + d ) = 4 1 e = 3 (a+b+c+d+e) - (a+b+c+d) = 4-1 \Rightarrow e = 3 ) to obtain the follwing results a = 2 , b = 1 , c = 0 , d = 2 , e = 3 a=2, \quad b=1, \quad c=0, \quad d=-2, \quad e=3 Therefore a e + b d + c = 2 3 + 1 2 + 0 = 8 + 1 = 9 a^e + b^d + c = 2^3 + 1^{-2} + 0 = 8 + 1 = \fbox{9}

Josh Rowley
Feb 4, 2014

If we sum all the equations then we get 4 ( a + b + c + d + e ) = 16 4(a+b+c+d+e) = 16 . Therefore a + b + c + d + e = 4 a+b+c+d+e = 4 . Subbing in the initial equations gives us 5 new equations: a + 2 = 4 a+2 = 4 b + 3 = 4 b+3 = 4 c + 4 = 4 c+4 = 4 d + 6 = 4 d+6 = 4 e + 1 = 4 e+1 = 4 Solving these gives us the solution ( 2 , 1 , 0 , 2 , 3 ) (2,1,0,-2,3) so a e + b d + c = 2 3 + 1 2 + 0 = 8 + 1 = 9 a^e+b^d+c = 2^3+1^{-2}+0= 8+1 = \fbox{9}

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