x(10-4.9x)=0
10x-4.9x^2=0
4.9x^2-10x=0
x^2-100/49x=0
x^2-2*50/49x+(50/49)^2=（50/49）^2
(x-50/49)^2=(50/49 )^2 x-50/49=±50/49 x1=0 x2=100/49