Tangent Ellipse 6E1360
1. **State the problem:** Find the equation of the tangent line to the ellipse $$4x^2 + 9y^2 = 40$$ that has a slope of $$-\frac{2}{9}$$.
2. **Recall the ellipse equation and implicit differentiation:** The ellipse is given by $$4x^2 + 9y^2 = 40$$.
To find the slope of the tangent line, differentiate implicitly with respect to $$x$$:
$$\frac{d}{dx}(4x^2) + \frac{d}{dx}(9y^2) = \frac{d}{dx}(40)$$
which gives
$$8x + 18y \frac{dy}{dx} = 0$$.
3. **Solve for $$\frac{dy}{dx}$$ (the slope of the tangent):**
$$18y \frac{dy}{dx} = -8x \Rightarrow \frac{dy}{dx} = -\frac{8x}{18y} = -\frac{4x}{9y}$$.
4. **Set the slope equal to the given slope:**
$$-\frac{4x}{9y} = -\frac{2}{9}$$
Multiply both sides by $$9y$$:
$$-4x = -2y$$
Simplify:
$$4x = 2y \Rightarrow y = 2x$$.
5. **Substitute $$y = 2x$$ into the ellipse equation to find the points of tangency:**
$$4x^2 + 9(2x)^2 = 40$$
$$4x^2 + 9 \times 4x^2 = 40$$
$$4x^2 + 36x^2 = 40$$
$$40x^2 = 40$$
$$x^2 = 1 \Rightarrow x = \pm 1$$.
6. **Find corresponding $$y$$ values:**
For $$x=1$$, $$y=2(1)=2$$.
For $$x=-1$$, $$y=2(-1)=-2$$.
7. **Write the tangent line equations using point-slope form:**
Slope $$m = -\frac{2}{9}$$.
For point $$(1,2)$$:
$$y - 2 = -\frac{2}{9}(x - 1)$$
Simplify:
$$y = -\frac{2}{9}x + \frac{2}{9} + 2 = -\frac{2}{9}x + \frac{20}{9}$$.
For point $$( -1, -2)$$:
$$y + 2 = -\frac{2}{9}(x + 1)$$
Simplify:
$$y = -\frac{2}{9}x - \frac{2}{9} - 2 = -\frac{2}{9}x - \frac{20}{9}$$.
**Final answer:** The equations of the tangent lines with slope $$-\frac{2}{9}$$ are
$$y = -\frac{2}{9}x + \frac{20}{9}$$ and $$y = -\frac{2}{9}x - \frac{20}{9}$$.