x①-2x②=a
x①+2x②=3a+1
x①*x②=2(a+1)
x①=2a+1/2
x②=a/2+1/4
x①*x②=2(a+1)=(2a+1/2)(a/2+1/4)
2(a+1)=(2a+1/2)(a/2+1/4)
2a+2=a²+a/4+a/2+1/8
16a+16=8a²+2a+4a+1
8a²-10a-15=0
a=[10±√(100+480)]/16=[10±√(580)]/16=[10±2√(145)]/16=[5±√(145)]/8
a①=(5+√145)/8
a②=(5-√145)/8