前面的没看明白,最后一道题：a^2-3a+1=0 a肯定不为0,两边同除以a,a+1/a=3
将a+1/a=3两边平方得到a^2+1/a^2=7,再次平方,a^4+1/a^4=47
①（2x+5b）（2x-b）②（x-2y+z）（2y-x-z）
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