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find the value

Source: saint petersburg 2015,grade 11,P1

March 14, 2016
algebra

Problem Statement

x,yx,y are real numbers such that x2+y2=1,20x315x=3x^2+y^2=1 , 20x^3-15x=3Find the value of 20y315y|20y^3-15y|.(K. Tyshchuk)