Conics again

Geometry Level 4

Find the area of the ellipse: 3 x 2 + 4 x y + 3 y 2 = 1 3x^2 +4xy +3y^2 =1 [take π = 22 7 \pi =\frac{22}{7} ]

22 7 \frac{22}{7} 11 7 × 7 \frac{11}{7 \times \sqrt{7}} 11 7 × 3 \frac{11}{7 \times \sqrt{3}} 22 7 × 5 \frac{22}{7 \times \sqrt{5}}

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2 solutions

Tanishq Varshney
Jan 29, 2015

If the centre of any conic is at the origin then the coefficient of x and y will be zero in its general equation .

Any point on ellipse is ( r cos θ , r sin θ ) (r\cos \theta,r\sin \theta)

After satisfying these coordinates in the given equation and on simplifying we get,

3 r 2 + 2 r 2 sin 2 θ = 1 3r^2 + 2r^2\sin 2\theta =1

r = 1 3 + sin 2 θ r= \frac{1}{\sqrt{3 +\sin 2\theta}} a = length of semi major axis b = length of semi minor axis

a = r m a x = 1 3 2 a=r_{max}=\frac{1}{\sqrt {3-2}} [ when denominator is minimum, r wil be maximum and minimum of sin 2 θ = 1 \sin 2\theta =-1

b = r m i n = 1 3 + 2 b=r_{min}=\frac{1}{\sqrt{3+2}} { maximum of sin 2 θ = 1 \sin 2\theta =1 ]

Area of ellipse = π a b \pi ab

= π 5 \frac{\pi}{\sqrt {5}}

Plz upvote if u liked the solution

It can be easily observed that the ellipse is symmetrical about x=y & x=-y .Rest is a piece of cake.

A Former Brilliant Member - 4 years, 3 months ago

A r e a o f a c o n i c s h a v i n g i t s c e n t e r a t o r i g i n i s o f t h e f o r m a x 2 + 2 h x y + b y 2 = 1. h 2 a b = 4 9 < 0 , w e c o n f i r m t h a t i t i s a n e l l i p s e . A n d s o , Area ~of ~a~ conics~ having~its~ center~ at~ origin~ is~ of~ the~ form ~~ax^2+2hxy+by^2=1.\\ h^2-ab=4-9<0,~we~ confirm~that~it~is~an~ellipse.~And~so,\\ i t s a r e a = 2 π 4 a b ( 2 h ) 2 = 2 22 7 4 a b ( 2 h ) 2 = 44 7 36 16 = 44 7 2 5 = 22 7 5 . its~area=\dfrac {2\pi}{\sqrt{4ab-(2h)^2}}=\dfrac {\frac {2*22} 7 }{\sqrt{4ab-(2h)^2}}=\dfrac {44}{7*\sqrt{36-16}}=\dfrac{44}{7*2*\sqrt5}=\dfrac{22}{7*\sqrt5}.

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