a+b=c+d,a3+b3=c3+d3(a+b)(a2-ab+b2)=(c+d)(c2-cd+d2)ab=cd (a+b)^2-4ab=(c+d)^2-4cd(a-b)^2=(c-d)^2a-b=c-d,或a-b=d-c2a=2c,2b=2d或2a=2d,2b=2c易证a1989+b1989=c1989+d1989