(1) We say that if a function at $x=a$ is differentiable, then there exists a tangent.
(2) But if a function have tangent at $x=a$, then it may not be differentiable at $x=a$.
Also, when secants are becoming parallel to each other as $h\rightarrow 0$, the secants approaches to a common tangent at that point. We use this condition to prove that a function is differentiable or not, i.e. LHD=RHD (have a tangent at that point). But in (2) , we have said that it may not be differentiable.
Make it clear to me ASAP.
Thanks in advance.
An endpoint can have a tangent but is not differentiable because the graph/function is not continuous at the endpoint.