tangent line vs secant line According to definition, secant lines intersect the curve on two different points say $P,Q$ while tangent lines intersect only at one point. Also according definition with $P$ fixed and $Q$ variable as $Q$ approaches $P$ along the curve direction of secant approaches that of tangent. 
Now my question is if is curve like sine curve then can we find a tangent line on arbitrary point if so i guess it will contradict the definition of tangent line that it only intersect only a single point of the curve
Please help 
Ahsan 

If it were correct to define the concept of tangent line by saying it's a line that intersects a curve at only one point, then the $y$-axis would be a tangent line to the curve $y=x^2$.  That is absurd.
