a^4-7a^3+5a^2+7a-6＝0→(a^4-2a^3+a^2)-(5a^3-10a^2+5a)-(6a^2-12a+6)＝0→a^2(a^2-2a+1)-5a(a^2-2a+1)-6(a^2-2a+1)＝0→(a^2-2a+1)(a^2-5a-6)＝0→(a-1)²(a+1)(a-6)＝0∴a1＝a2＝1,a3＝-1,a4＝6....