解：
a²+3a+1＝（a+1）^2 +a
（a+1）^2 +a+1 = 1 设 a+1 = x x^2 +x -1 =0 x = [-1 +/-根号下1-4*1*(-1)]/2 =
(-1+/-根号5）/2
a+1 = (-1+/-根号5）/2
