A geometry problem by Yahia El Haw

Geometry Level 4

In the figure given below M L ML and N L NL are adjacent sides of a square and the arc M P N MPN is drawn with L L as centre and L M LM as radius. P P is a point on the arc and P Q R S PQRS is a square such that, R S RS , if extended, passes through N N while R Q RQ , if extended, passes through M M . What is the ratio of the area of a square of side M L ML and the square P Q R S PQRS ?

3 : ( 3 2 2 ) 3 : (3 - 2\sqrt { 2 } ) 4 : ( 3 2 2 ) 4 : (3 - 2\sqrt { 2 } ) 2 : ( 3 2 2 ) 2 : (3 - 2\sqrt { 2 } ) 1 : ( 3 2 2 ) 1 : (3 - 2\sqrt { 2 } )

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3 solutions

Yahia El Haw
Jan 18, 2017

The correct answer is 2 : ( 3 2 2 ) 2 : (3 - 2\sqrt { 2 } ) It is given that R S RS passes through L L and R Q RQ passes through M M .
On drawing the line R S L RSL and R Q M RQM , the diagram shown above results. From the diagram, it is clear that M N L R MNLR has to be a square and P P has to be the mid-point of the arc.

As P P is the mid-point of the arc, R N RN passes through P P ,

R P RP = a ( 2 1 ) a (\sqrt { 2 } - 1)

where M N MN = P N PN = a a

\Rightarrow P Q PQ = R P 2 \frac { RP }{ \sqrt { 2 } }

= a ( 2 1 ) / 2 a(\sqrt { 2 } -1)/\sqrt { 2 }

M N 2 : P Q 2 = 2 : 3 2 2 { MN }^{ 2 } : { PQ }^{ 2 }=2 : 3-2\sqrt { 2 }

Please check your letters. Do you mean N in place of L in the first line ? Similar afterwards..

Niranjan Khanderia - 4 years, 4 months ago

Let the side length of square M L N R MLNR be X X and the side length of square Q P S R QPSR be Y Y , then R L = X 2 RL=X\sqrt{2} and R P = Y 2 RP=Y\sqrt{2} .

R L = R P + X RL=RP+X \implies X 2 = Y 2 + X X\sqrt{2}=Y\sqrt{2}+X \implies Y 2 = X 2 X Y\sqrt{2}=X\sqrt{2}-X \implies Y = X 2 X 2 Y=\dfrac{X\sqrt{2}-X}{\sqrt{2}}

Squaring both sides, we have Y 2 = X 2 ( 3 2 2 ) 2 Y^2=\dfrac{X^2(3-2\sqrt{2})}{2} .

Finally, the ratio of the areas is X 2 Y 2 = X 2 X 2 ( 3 2 2 ) 2 = 2 3 2 2 \dfrac{X^2}{Y^2}=\dfrac{X^2}{\dfrac{X^2(3-2\sqrt{2})}{2}}=\dfrac{2}{3-2\sqrt{2}} and the desired answer is 2 : 3 2 2 \boxed{2:3-2\sqrt{2}}

If LP= r is the radius of the circle, LR the diagonal of LMRN= 2 \sqrt2 r and PR=RL-PL, the diagonal of PQRS = ( 2 \sqrt2 -1)r.
So the ratio= { 2 2 1 } 2 2 : 3 2 . \left\{\dfrac{\sqrt2}{\sqrt2-1}\right \}^2\ \implies\ 2 :\ 3-\sqrt2.

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