I don't think I can do it

Calculus Level 5

Find the area of the figure bounded by the curve x 3 + y 3 = 1729 x y x^3+y^3=1729xy .

If the answer comes as A 2 B \frac{A^2}{B} where A A and B B are co-prime positive integers. Find A + B A+B .


The answer is 1735.

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3 solutions

Otto Bretscher
Dec 5, 2015

This is a Folium of Descartes , x 3 + y 3 = 3 a x y x^3+y^3=3axy , with a = 1729 3 a=\frac{1729}{3} . The loop C C of this curve (i.e., the part in the first quadrant) can be parametrized by x = 3 a t 1 + t 3 , y = 3 a t 2 1 + t 3 x=\frac{3at}{1+t^3}, y=\frac{3at^2}{1+t^3} for t 0 t\geq 0 . Thus the area enclosed by C C is C x d y = 9 a 2 0 t 2 ( t 3 2 ) ( 1 + t 3 ) 3 d t = 9 a 2 [ 1 2 t 3 6 ( 1 + t 3 ) 2 ] 0 = 3 a 2 2 = 172 9 2 6 \int_{C}xdy=-9a^2\int_{0}^{\infty}\frac{t^2(t^3-2)}{(1+t^3)^3}dt=-9a^2\left[\frac{1-2t^3}{6(1+t^3)^2}\right]_{0}^{\infty}=\frac{3a^2}{2}=\frac{1729^2}{6}

The sum we seek is 1735 \boxed{1735}

I like your solution very much! I got a way of deriving the parametrization that you used before, by converting the equation x 3 + y 3 = 3 a x y x^3+y^3=3axy into polar coordinates first, so we get r = 3 a sin θ cos θ cos 3 θ + sin 3 θ . r=\frac{3a \sin\theta \cos \theta}{\cos^3 \theta +\sin^3\theta}. for 0 θ π 2 0\leq \theta\leq \frac{\pi}{2} . Then using the equations x = r cos θ , y = r sin θ , x=r\cos \theta, \:y=r\sin\theta, we obtain x = 3 a sin θ cos 2 θ cos 3 θ + sin 3 θ x= \frac{3a \sin\theta \cos^2 \theta}{\cos^3 \theta +\sin^3\theta} and y = 3 a sin 2 θ cos θ cos 3 θ + sin 3 θ y=\frac{3a \sin^2\theta \cos \theta}{\cos^3 \theta +\sin^3\theta} Then dividing in each fraction the denominator and numerator by cos 3 θ \cos^3\theta . Then we get x = 3 a tan θ 1 + tan 3 θ x= \frac{3a \tan\theta }{1 +\tan^3\theta} and y = 3 a tan 2 θ 1 + tan 3 θ y=\frac{3a \tan^2\theta }{1+\tan^3\theta} Now you can get the parametrization by making t = tan θ , t=\tan \theta, and of course when 0 θ < π 2 , 0\leq\theta<\frac{\pi}{2}, we have 0 t < . 0\leq t<\infty. Is there any other way of deriving it?

Arturo Presa - 5 years, 6 months ago

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This is a very lucid and elegant explanation, as one would expect from a Cuban- and Soviet-trained mathematician. Thank you!

Otto Bretscher - 5 years, 6 months ago

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Otto, I really appreciate you compliment, but, believe me, I am nothing compared to you. I am always learning from your solutions, and I am being sincere: you are a great professor and mathematician.

Arturo Presa - 5 years, 6 months ago

@Otto Bretscher sir... Well I didn't do this problem complete but I sought it(I am lazy at calculations sorry!!) As we know that the curve is symmetric about line y=x we can apply transformation on y=(X+Y)/√2 and x=(X-Y)/√2 which gave us an explicit function of Y in terms of X and 2 times the upper bound branch with the new horizontal axis would give the answer.

EDIT-the integral is solvable,it needs calculations and now I got the answer 1735.

Righved K - 5 years, 6 months ago

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Yes, you can do it this way, but the computations are quite tedious.

Otto Bretscher - 5 years, 6 months ago

What r u studying now and when we ll get all these in maths

Suneel Kumar - 5 years, 6 months ago

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I had done a normal jee prep and started using the concepts...mathematics is the subject which I love the most

Righved K - 5 years, 6 months ago

Nice sir!!! We've got similar solutions.. This is a very nice problem.

Jun Arro Estrella - 5 years, 6 months ago
Arturo Presa
Dec 8, 2015

We can also solve this problem by converting the equation x 3 + y 3 = 3 a x y x^3+y^3=3axy into polar coordinates. So we get r = 3 a sin θ cos θ cos 3 θ + sin 3 θ . r=\frac{3a \sin\theta \cos \theta}{\cos^3 \theta +\sin^3\theta}. Dividing both the numerator and denominator by cos 3 θ \cos^3\theta we obtain that r = 3 a sec θ tan θ 1 + tan 3 θ . r=\frac{3a \sec\theta \tan \theta}{1 +\tan^3\theta}. Now the given area can be expressed as A = 1 2 0 π / 2 r 2 d θ = 1 2 0 π / 2 9 a 2 sec 2 θ tan 2 θ ( 1 + tan 3 θ ) 2 d θ = 9 a 2 6 0 π / 2 d tan 3 θ ( 1 + tan 3 θ ) 2 A=\frac{1}{2}\int_0^{\pi/2}r^2d\theta=\frac{1}{2}\int_0^{\pi/2}\frac{9a^2 \sec^2\theta \tan^2 \theta}{(1 +\tan^3\theta)^2}d\theta =\frac{9a^2}{6}\int_0^{\pi/2}\frac{d \tan^3\theta }{(1 +\tan^3\theta)^2} = 3 a 2 2 1 1 + tan 3 θ 0 π / 2 = 3 a 2 2 . =-\frac{3a^2}{2}\frac{1}{1+\tan^3\theta}\bigg|_0^{\pi/2}=\frac{3a^2}{2}. In this case a = 1729 3 , a=\frac{1729}{3}, so the area A = 172 9 2 6 , A=\frac{1729^2}{6}, and, therefore, the answer is 1735. 1735.

Andreas Wendler
Dec 27, 2015

You can also get the solution by usage of the formula for the cubic equation in y. This brings y 1 ( x ) , y 2 ( x ) y_{1}(x), y_{2}(x) , i.e. the two curves including the area wanted. After determination of maximum x to 914.872 by setting y 1 ( x ) = y 2 ( x ) y_{1}(x) = y_{2}(x) you have the integration range to numerically calculate the integral of the area v between the two curves delivering v = 498240.1666 => 6 v = 172 9 2 6v = 1729^{2} .

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