Given, OP = $26cm$

PT = tangent length = $10cm$

To find: radius = OT =$?$

We know that,

Radius and tangent are perpendicular at the point of contact, $\angle OTP={{90}^{\circ }}$ $$

So, $\vartriangle OTP$  is right angled triangle.

Then by Pythagoras theorem, we have

$O{{P}^{2}}=O{{T}^{2}}+P{{T}^{2}}$

${{26}^{2}}=O{{T}^{2}}+{{10}^{2}}$

$O{{T}^{2}}=676-100$

$OT=\sqrt{576}$

$OT=24cm$

Therefore, OT = radius = $24cm$