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2x²+5x-1=0
delta=25+4×2
delta=33
x1=(-5-rad33)/4
x2=(-5+rad33)/4
x1×x2=(-5-rad33)(-5+rad33)/16
x1×x2=[(-5)²-(rad33)²] :16
x1×x2=(25-33)/16
x1×x2=-8/16
x1×x2=-1/2