A curve is given by the parametric equations , , where the parameter varies over all non negative real numbers. Find the number of straight lines passing through the point which are tangent to the curve.
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Taking d t d y = 2 t − 1 and d t d x = 2 t + 1 , the slope of y ( x ) can be computed per d x d y = d t d y / d t d x = 2 t + 1 2 t − 1 . The tangent line for all points ( x ( t ) , y ( t ) ) on this curve is expressible as:
y − ( t 2 − t + 1 ) = ( 2 t + 1 2 t − 1 ) ⋅ ( x − ( t 2 + t + 1 ) )
and if this tangent line is to pass through the point ( 1 , 1 ) , then we obtain:
1 − ( t 2 − t + 1 ) = ( 2 t + 1 2 t − 1 ) ⋅ ( 1 − ( t 2 + t + 1 ) ) ;
or t − t 2 = ( 2 t + 1 2 t − 1 ) ⋅ ( − t 2 − t ) ;
or 1 − t = − ( 2 t + 1 2 t − 1 ) ⋅ ( 1 + t ) ;
or ( t − 1 ) ( 2 t + 1 ) = ( 2 t − 1 ) ( t + 1 ) ;
or 2 t 2 − t − 1 = 2 t 2 + t − 1 ;
or − t = t ;
or t = 0 ⇒ ( x ( 0 ) , y ( 0 ) ) = ( 1 , 1 ) . Hence, there is only O N E such tangent line that satisfies the above initial parametric conditions.