用反证法.假设a²+b²+c²+d²+ab+cd = 1.则(a+b)²+(c+d)²+(a-d)²+(b+c)² = 2(a²+b²+c²+d²+ab+cd)-2(ad-bc) = 0.由a,b,c,d都为实数,只有a+b = c+d = a-d =...