{ x 8 − 2 8 y 2 x 6 + 7 0 x 4 y 4 − 2 8 y 6 x 2 + y 8 = 2 3 8 y x 7 − 5 6 y 3 x 5 + 5 6 y 5 x 3 − 8 y 7 x = 2 1 How many pairs of ( x , y ) satisfy this system of equation above?
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You forgot to include k=0 (still 8 pairs, you just forgot to write it).
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Plus two sides of the equation we get: x 8 − 2 8 y 2 x 6 + 7 0 x 4 y 4 − 2 8 y 6 x 2 + y 8 + i ( 8 y x 7 − 5 6 y 3 x 5 + 5 6 y 5 x 3 − 8 y 7 x ) = 2 3 + i 2 1 ⇔ ( x + i y ) 8 = 2 3 + i 2 1 = c o s 6 π + i s i n 6 π ⇔ x + i y = c o s 8 6 π + k 2 π + i s i n 8 6 π + k 2 π ( k = 0 , 1 , 2 , . . . , 7 ) This system of equation have 8 pairs: ( x , y ) = ( c o s 8 6 π + k 2 π ; s i n 8 6 π + k 2 π ) , ( k = 0 , 1 , 2 , . . 7 ) So the answer is 8 PAIRS.