L=U²A divided by (A+B) ² make A the subject of formular

A = [ - 2Bl + U^2 + / - √ ((2BL - U^2) ^2 - 4B^2L^2) ] / 2L

Step-by-step explanation:

L = U^2A / (A + B) ^2

U^2 A = L (A^2 + 2AB + B^2)

U^2 A = LA^2 + 2ABL + B^2L

LA^2 + 2ABL - U^2A + B^2 L = 0

LA^2 + (2BL - U^2) A + B^2L = 0

Using the quadratic formula

A = [ - (2BL - U^2) + / - √[ (2BL - U^2) ^2 - 4*L*B^2L] / 2L

A = [ - 2BL + U^2 + / - √[ (2BL - U^2) ^2 - 4B^2L^2) ] / 2L