Question 29$AB$ is a diameter of a circle. Tangents $AD$ and $BC$ are drawn so that $AC$ and $BD$ intersect in a point on the circle. If $AD=a$ and $BC=b,$ $a\ne b,$ the diameter of the circle is:$\lvert a-b\rvert$$\dfrac12(a+b)$$\sqrt{ab}$$\dfrac{ab}{a+b}$$\dfrac12\cdot\dfrac{ab}{a+b}$