Rui is a professional deep water free diver. His altitude (in meters relative to sea level), xxx seconds after diving, is modeled by: d(x)=\dfrac12x^2 -10xd(x)= 2 1 ​ x 2 −10xd, left parenthesis, x, right parenthesis, equals, start fraction, 1, divided by, 2, end fraction, x, squared, minus, 10, x How many seconds after diving will Rui reach his lowest altitude?