ab=[(a+b)^2-a^2-b^2]/2=(1-2)/2=-1/2,a^4+b^4=(a^2+b^2)^2-2a^2b^2=4-2*(-1/2)^2=7/2
(a+b)²=a²+b²+2ab=1ab=(1-2)/2=-1/2
ab=-1
