(1)36x^2-1=0; x^2=1/36 x=±1/6 (2)4x^2=81; x^2=81/4 x=±9/2 (3)(x+5)^2=25 x+5=±5 x1=-10,x2=0 (4)x^2+2x+1=4. （x+1)^2=4 x+1=±2 x1=-3,x2=1
