(M+1)^7ç³»æ° 1,7,21,35,35,21,7,1
=>(ax+1)^7=(ax)^7 +7(ax)^6+21(ax)^5+35(ax)^4+35(ax)^3+21(ax)^2+7(ax)+1
x^3çç³»æ°æ¯35*a^3 , x^2çç³»æ°21*a^2 , x^4çç³»æ°35*a^4
=> 2(35*a^3)=(21*a^2)+(35*a^4) æ¶å»7a^2
=>10a=3+5a^2 => 5a^2-10a+3=0
=>a=(5屉10)/5
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