Question 427559
original speed = x
increased speed = x+100

...
Original time = 2800/x

Time after speed increase = 2800/(x+100)

2800/x - 2800/(x+100)=1/2

LCD = x(x+100)
multiply by x(x+100)
2800(x+100)-2800*x= 1/2* x(x+100)
2800x+280000-2800x=1/2(x^2+100x)
5600x+560000-5600x= x^2+100x
x^2+100x-560000=0
Find the roots of the equation by quadratic formula					
					
a=	1	,b=	100	,c=	-560000
					
b^2-4ac=	10000	+	2240000		
b^2-4ac=	2250000				
{{{x=(-b+-sqrt(b^2-4ac))/(2a)}}}					
{{{x1=(-b+sqrt(b^2-4ac))/(2a)}}}					
{{{x1=(-2+sqrt(664))/(2)}}}					
x1=(	-100	+	1500	)/	2
x1=	700				
x2=(	-100	-1500	) /	2	
x2= 	-800				
Ignore negative value					
plane speed	=	700	mph		
New plane speed = 800 mph