Olympiad corner

Algebra Level 5

{ x 8 28 y 2 x 6 + 70 x 4 y 4 28 y 6 x 2 + y 8 = 3 2 8 y x 7 56 y 3 x 5 + 56 y 5 x 3 8 y 7 x = 1 2 \left\{\begin{matrix} x^8-28y^2x^6+70x^4y^4-28y^6x^2+y^8=\frac{\sqrt{3}}{2} & & \\ 8yx^7-56y^3x^5+56y^5x^3-8y^7x=\frac{1}{2} & & \end{matrix}\right. How many pairs of ( x , y ) (x,y) satisfy this system of equation above?


The answer is 8.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Son Nguyen
Jan 24, 2016

Plus two sides of the equation we get: x 8 28 y 2 x 6 + 70 x 4 y 4 28 y 6 x 2 + y 8 + i ( 8 y x 7 56 y 3 x 5 + 56 y 5 x 3 8 y 7 x ) = 3 2 + i 1 2 x^8-28y^2x^6+70x^4y^4-28y^6x^2+y^8+i(8yx^7-56y^3x^5+56y^5x^3-8y^7x)=\frac{\sqrt{3}}{2}+i\frac{1}{2} ( x + i y ) 8 = 3 2 + i 1 2 = c o s π 6 + i s i n π 6 \Leftrightarrow (x+iy)^8=\frac{\sqrt{3}}{2}+i\frac{1}{2}=cos\frac{\pi }{6}+isin\frac{\pi }{6} x + i y = c o s π 6 + k 2 π 8 + i s i n π 6 + k 2 π 8 ( k = 0 , 1 , 2 , . . . , 7 ) \Leftrightarrow x+iy=cos\frac{\frac{\pi}{6}+k2\pi }{8}+isin\frac{\frac{\pi }{6}+k2\pi }{8}(k=0,1,2,...,7) This system of equation have 8 pairs: ( x , y ) = ( c o s π 6 + k 2 π 8 ; s i n π 6 + k 2 π 8 ) , ( k = 0 , 1 , 2 , . . 7 ) (x,y)=(cos\frac{\frac{\pi}{6}+k2\pi}{8};sin\frac{\frac{\pi}{6}+k2\pi}{8}),(k=0,1,2,..7) So the answer is 8 PAIRS.

You forgot to include k=0 (still 8 pairs, you just forgot to write it).

Mark Gilbert - 5 years, 4 months ago

Log in to reply

Oh thanks.Yeap

Son Nguyen - 5 years, 4 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...