(1)x²-14x+7²=49
(x-7)²=49
x-7=±7
x₁=14、x₂=0
(3)x²-x+(1/2)²=56+1/4
(x-1/2)²=225/4
x-1/2=±15/2
x₁=8、x=₂=-7
(5)4x²-31x=45
x²-31/4x=45/4
x²-31/4x+(31/8)²=45/4+961/64
(x-31/8)²=1681/64
x-31/8=±41/8
x₁=9、x₂=-5/2
(7)x²+9x+8=-12
x²+9x+(9/2)²=-20+81/4
(x+9/2)²=1/4
x+9/2=±1/2
x₁=-4、x₂=-5