Geometry?

Geometry Level 5

Find the minimum value of

x 2 + ( 20 y ) 2 + y 2 + ( 21 z ) 2 + z 2 + ( 20 w ) 2 + w 2 + ( 21 x ) 2 . \sqrt{x^2+(20-y)^2}+\sqrt{y^2+(21-z)^2}+\sqrt{z^2+(20-w)^2}+\sqrt{w^2+(21-x)^2}.


The answer is 58.

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2 solutions

Minimum value is the length of A B AB , that is 4 2 2 + 4 0 2 = 58 \sqrt{42^2+40^2}=58

what will be x,y,z,w when it is 58?

Le Tran Duy Anh - 3 years, 12 months ago

how does someone even think of something like this..?

chris recupero - 3 years, 11 months ago

Why not interchange X and w. That way we get 2*41^2. Plz explain

suraj sahoo - 3 years, 1 month ago

Differentiating the function partially w.r.t. each variable and equating to 0, we get the value of the variables as,
X = Z = 21 2 , W = Y = 10. X=Z=\frac{21} 2,\ \ \ W=Y=10.\\
Substituting these values in the expression,
w e g e t ( 21 2 ) 2 + 100 + 100 + ( 21 2 ) 2 + ( 21 2 ) 2 + 100 + 100 + ( 21 2 ) 2 = 4 ( 100 + ( 21 2 ) 2 = 58 we\ get\ \ \sqrt{( \dfrac {21} 2 )^2 + 100 } +\sqrt{100+(\dfrac {21} 2 )^2 }+\sqrt{(\dfrac {21} 2 )^2 + 100}+\sqrt{100+(\dfrac {21} 2 )^2 }\\ =4*\sqrt{(100+(\dfrac {21} 2 )^2 }= \color{#D61F06}{58}


Why have you taken w= 10 ? , to get min. value we should take w=20 and thus we get 54 as minimum value all other variable has same value as suggested by you. if i am wrong please correct me

Shiva Gupta - 5 years ago

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