How to solve the following problem?

Area of triangle = s(s-a)(s-b)(s-c)

where s = a+b+c/2 and a = 60 b=80 and c=100

Now area of the field which the horses cannot graze is given by,

Area of ABC - (Area of sector1 + Area of sector 2 + Area of sector 3)...................eqn1

Now ABC = 90⁰ , since the sides form a pythagorain triplet.

AB = a = 60

BC = b = 80

AC = c= 100

Now we have to ACB and BAC

Let ACB = θ₁ and BAC = θ₂

So, tan θ₁ = AB/BC = 6/8

θ₁ = tan^-1 (3/4)        find the value from the trigonimetric table.

Similarly by using trigonimetric ratio's compute θ₂ 

Compute area of Sector by using the formula Πr²θ/360^o

Put the values of area of sectors in equation 1 to get the final answer.