Parametric curve tangency

Calculus Level 3

A curve is given by the parametric equations x = t 2 + t + 1 x=t^2+t+1 , y = t 2 t + 1 y=t^2-t+1 , where the parameter t t varies over all non negative real numbers. Find the number of straight lines passing through the point ( 1 , 1 ) (1,1) which are tangent to the curve.

3 4 2 1

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1 solution

Tom Engelsman
Dec 10, 2020

Taking d y d t = 2 t 1 \frac{dy}{dt} = 2t-1 and d x d t = 2 t + 1 \frac{dx}{dt} = 2t+1 , the slope of y ( x ) y(x) can be computed per d y d x = d y d t / d x d t = 2 t 1 2 t + 1 . \frac{dy}{dx} = \frac{dy}{dt} / \frac{dx}{dt} = \frac{2t-1}{2t+1}. The tangent line for all points ( x ( t ) , y ( t ) ) (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 ) ) y - (t^2-t+1) = ( \frac{2t-1}{2t+1}) \cdot (x - (t^2+t+1))

and if this tangent line is to pass through the point ( 1 , 1 ) (1,1) , then we obtain:

1 ( t 2 t + 1 ) = ( 2 t 1 2 t + 1 ) ( 1 ( t 2 + t + 1 ) ) 1 - (t^2-t+1) = ( \frac{2t-1}{2t+1}) \cdot (1 - (t^2+t+1)) ;

or t t 2 = ( 2 t 1 2 t + 1 ) ( t 2 t ) t-t^2 = ( \frac{2t-1}{2t+1}) \cdot (-t^2-t) ;

or 1 t = ( 2 t 1 2 t + 1 ) ( 1 + t ) 1-t = - ( \frac{2t-1}{2t+1}) \cdot (1+t) ;

or ( t 1 ) ( 2 t + 1 ) = ( 2 t 1 ) ( t + 1 ) ; (t-1)(2t+1) = (2t-1)(t+1);

or 2 t 2 t 1 = 2 t 2 + t 1 ; 2t^2 -t-1 = 2t^2 + t -1;

or t = t ; -t = t;

or t = 0 ( x ( 0 ) , y ( 0 ) ) = ( 1 , 1 ) \boxed{t=0 \Rightarrow (x(0),y(0)) = (1,1)} . Hence, there is only O N E \boxed{ONE} such tangent line that satisfies the above initial parametric conditions.

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