(1/2)x×(x+1+x+0.4)=0.78
(1/2)x×(2x+1.4)=0.78
x^2+0.7x-0.78=0
(x-0.6)(x+1.3)=0
x-0.6=0,x+1.3=0
x=0.6,x=-1.3