Meany straight lines ||

Geometry Level 3

The equation of a pair of straight lines is -

A x 2 + H y 2 + 2 G x y = 32 Ax^2+Hy^2+2Gxy=32

Where, A=Arithmetic mean of a & b, G=geometric mean of a & b, H=harmonic mean of a & b.(A and G 0 \neq0 )

If H = 2 H=2 ,

What is the distance between the Y-intercepts made by the two lines?


This is an original problem and belongs to my set Raju Bhai's creations

8 32 16 0

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2 solutions

Rajath Rao
Nov 16, 2017

A=Arithmetic mean of a & b, G=geometric mean of a & b, H=harmonic mean of a & b.

We know that , G = A H G=\sqrt{AH}

Given equation,

A x 2 + H y 2 + 2 G x y = 32 \implies Ax^2+Hy^2+2Gxy=32

A x 2 + H y 2 + 2 A H x y = 32 \implies Ax^2+Hy^2+2\sqrt{AH}xy=32 . …(because G = A H G=\sqrt{AH} )

( A x ) 2 + ( H y ) 2 + 2 A H x y = 32 \implies (\sqrt{A}x)^2+(\sqrt{H}y)^2+2\sqrt{A}\sqrt{H}xy=32 ……… (this is in form of x 2 + y 2 + 2 x y = ( x + y ) 2 x^2+y^2+2xy=(x+y)^2 )

( A x + H y ) 2 = 32 \implies (\sqrt{A}x+\sqrt{H}y)^2=32

A x + H y = ± 32 \implies \sqrt{A}x+\sqrt{H}y=\pm\sqrt{32}

y = A H x ± 32 H \implies y=-\sqrt{\frac{A}{H}}x\pm\sqrt{\frac{32}{H}}

This is in form of y = m x + c y=mx+c

So, the intercepts are + 32 H +\sqrt{\frac{32}{H}} and 32 H -\sqrt{\frac{32}{H}}

Given, H=2

Y- intercept points are (0,4) and (0,-4)

Distance between the intercepts is 8 \boxed{8}

Skanda Prasad
Nov 15, 2017

Let a = b a=b (Considering the equality case)

A = 2 a 2 = a A=\dfrac{2a}{2}=a

G = a 2 = a G=\sqrt{a^2}=a

H = 2 a 2 2 a = a H=\dfrac{2a^2}{2a}=a

But given that H = 2 H=2

A = G = H = 2 \implies A=G=H=2

Hence, the combined equation becomes 2 x 2 + 2 y 2 + 4 x y = 32 2x^2+2y^2+4xy=32 .

Which is the same as writing x 2 + y 2 + 2 x y = 16 x^2+y^2+2xy=16

( x + y ) 2 = 16 \implies (x+y)^2=16

( x + y ) = ± 4 \implies (x+y)=\pm4

Therefore, the two separated equations are x + y = 4 x+y=4 and x + y = 4 x+y=-4 .

Clearly intercepts are ( 0 , 4 ) (0,4) and ( 0 , 4 ) (0,-4) .

Distance between them, d d = 8 \boxed{8}

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