Answer:
Step-by-step explanation:
I honestly have no idea what you mean by answer by formula, but I'm going to give it my best. I began by squaring both sides to get:
(a² - b²) tan²θ = b² and then distributed to get:
a² tan²θ - b² tan²θ = b² and then got the b terms on the side to get:
a² tan²θ = b² + b² tan²θ and then changed the tans to sin/cos to get:
 and isolated the sin-squared on the left to get:
 and isolated the sin-squared on the left to get:
 and distributed to get:
 and distributed to get:
*** *** and factored the right side to get:
*** and factored the right side to get:
 and utilized a trig Pythagorean identity to get:
 and utilized a trig Pythagorean identity to get:
 and then solved for sinθ in the following way:
 and then solved for sinθ in the following way:
 so
 so
 This, along with the *** expression above will be important. I'm picking up at the *** to solve for cosθ:
 This, along with the *** expression above will be important. I'm picking up at the *** to solve for cosθ:
 and get the cos²θ alone on the right by subtracting to get:
 and get the cos²θ alone on the right by subtracting to get:
 and divide by b² to get:
 and divide by b² to get:
 and factor on the left to get:
 and factor on the left to get:
 and take the square root of both sides to get:
 and take the square root of both sides to get:
 and simplify to get:
 and simplify to get:
 and go back to the identity we found for sinθ and sub it in to get:
 and go back to the identity we found for sinθ and sub it in to get:
 and simplifying a bit gives us:
 and simplifying a bit gives us:

That's my spin on things....not sure if it's what you were looking for. If not.....YIKES