Question 183980
540/(x+15)+540/(x-15)=8.8
[540(x-15)+540(x+15)]/(x+15)(x-15)=8.8
[540x-8100+540x+8100]/(x^2-225)=8.8
1080x/(x^2-225)=8.8
1080x=8.8x^2-1980
8.8x^2-1080x-1980=0
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
x=(1080+-sqrt[-1080^2-4*8.8*-1980])/2*8.8
x=(1080+-sqrt[1,166,400+69,696]/17.6
x=(1080+-sqrt1,236,096)/17.6
x=(1080+-1,111.7985)/17.6
x=(1080+1,111.7985)/17.6
x=2,191.7985/17.6
x=124.53 mph. is the epeed of the plane in still air.