If the harmonic mean between the roots of $(5+ \sqrt 2)x^2-bx+(8+ 2\sqrt 5)=0$ is $4$, then the value of b

$(a)\;2 \quad (b)\;3 \quad (c)\;4-\sqrt 5 \quad (d)\;4+\sqrt 5$

(d) $4 +\sqrt 5$
answered Nov 7, 2013 by