solid angle

Ω=ds₁/r₁²=ds₂/r₂²=ds₂/r₂²=ds₃/r₃²=ds_n/r_n²=SOLID ANGLE.
SOLID ANGLE=area/(distance)²

total solid angle =4πr²/r²=4π

dΩ=ds\ r² =ds cosθ/r²


 The total angle subtended by every element with or under the surface will be the integral of dΩ(=4π) with respect to Ω.