# Constructing the tangent line of circle without calculus

The tangent line of a circle can be defined as a line that intersects the circle at one point only.

Put a circle in the rectangular coordinate system.

Let $(x_0, y_0)$ be a point on a circle. The tangent line at $(x_0, y_0)$ is a line intersects the circle at $(x_0, y_0)$ only.

Let’s first find a function $y=kx+m$ that represents the line.

From circle’s equation $x^2+y^2=r^2$, we have

$y^2=r^2-x^2$

Since the line intersects the circle at $(x_0, y_0)$ only,

$r^2-x^2=(kx+m)^2$

has only one solution.

That means

$k^2x^2+x^2+2kmx+m^2-r^2 =0$

has only one solution. i.e., its discriminant

$(2km)^2-4(k^2+1)(m^2-r^2)=0\quad\quad\quad(1)$

By the definition of slope,

$kx+m-y_0 = k(x-x_0)$.

It follows that

$m =y_0-kx_0\quad\quad\quad(2)$

Substitute (2) into (1) and solve for $k$ gives

$k = \frac{-x_0}{y_0}\quad\quad\quad(3)$

The slope of line connecting $(0, 0)$ and $(x_0, y_0)$ where $x_0 \neq 0$ is $\frac{y_0}{x_0}$.

Since $\frac{-x_0}{y_0}\cdot \frac{y_0}{x_0} = -1$, the tangent line is perpendicular to the line connecting $(0, 0)$ and $(x_0, y_0)$.

Substitute (3) into $y = k x +m$, we have

$y=-\frac{x_0}{y_0} x + m\quad\quad\quad(4)$.

The fact that the line intersects the circle at $(x_0, y_0)$ means

$y_0 = -\frac{x_0^2}{y_0} + m$

or

$y_0^2=-x_0^2+ my_0$.

Hence,

$m =\frac{x_0^2+y_0^2}{y_0} = \frac{r^2}{y_0}$.

It follows that by (4),

$x_0 x +y_0 y = r^2\quad\quad\quad(5)$

(5) is derived under the assumption that $y_0 \neq 0$. However, by letting $y_0 =0$ in (5), we obtain two tangent lines that can not be expressed in the form of $y=kx+m$:

$x=-r, x=r$

# Constructing the tangent line of quadratic without calculus

The tangent line of a quadratic function at $(x_0, y_0)$is a line $y=kx+m$ that intersects $y=ax^2+bx+c$ at $(x_0, y_0=ax_0^2+bx_0+c)$ only.

The presence of function $y=kx+m$ immediately excludes the vertical line $x=x_0$ which also intersects $y=ax^2+bx+c$ at $(x_0, ax_0^2+bx_0+c)$ only (see Fig. 1).

Fig. 1

Let’s find $k$.

Line $y = kx+m$ intersects $y=ax^2+bx+c$ at $(x_0, ax_0^2+bx_0+c)$ only means quadratic equation

$ax^2+bx +c =kx +m$

has only one solution. That is, the discriminant of $ax^2+bx+c-kx-m =0$ is zero:

$(b-k)^2-4a(c-m) = 0\quad\quad\quad(1)$

Fig. 2

And, by the definition of slope (see Fig. 2),

$(x-x_0)k = (kx+m)-(ax_0^2+bx_0+c)$.

It follows that

$m = (ax_0^2+b_0+c)-x_0 k\quad\quad\quad(2)$

Substituting (2) into (1), we have

$(b-k)^2-4a(c-((a_0 x^2+b x_0 + c)-x_0 k)=0$.

Solve it for $k$ gives

$k = 2 a x_0 +b$.

Fig. 3