Is there a way to isolate variable U? (or will i have to solve numerically):

1 - SQRT[ 1 + (a^2 - a*u^2)/(b^2 - u^2) ] =
SQRT[ (b^2 - a^2)/( b^2 - u^2) ]

Only u is a variable, both a and b are constants. This is a simplified version of the problem, with u being a binomial and constants a and b are related in equation a^2+c^2 = b^2.