If \( f(x) = \int \frac{5x^8 + 7x^6}{(x^2 + 1 + 2x^7)^2} \, dx , \; (x \geq 0), \; f(0) = 0 \) and \( f(1) = \frac{1}{k} \), then the value of \( K \) is ____________.