# Calculus Questions

## Answer

**1) **You have a curve defined by

$$ x - y^3 = 2 x y, $$**a) **Let's apply implicit differentiation

$$ \blue{ \frac{d}{dx}} x - \blue{ \frac{d}{dx}} y^3 = \blue{ \frac{d}{dx}}2 x y $$

$$ 1 - 3 y^2 \blue{ \frac{d y}{dx}} = 2 \left( y + x \blue{ \frac{d y}{dx}} \right)$$

$$ 1 - 3 y^2 \blue{ \frac{d y}{dx}} = 2 \left( y + x \blue{ \frac{d y}{dx}} \right)$$

$$ \blue{ \frac{d y}{dx}} \left( -3 y^2 - 2 x \right) = \left( 2y - 1 \right)$$

$$ \blue{ \frac{d y}{dx}} = g(x,y) = \frac{ 1-2y }{ 3 y^2 + 2 x }$$**b)** To determine the tangent to this curve at the point (-1,1),

The tangent line equation at a point $P:(x_0,y_0)$ is given by,

$$ y -y_0 = m (x - x_0), \qquad m = \frac{d y}{dx}\bigg|_{x=x_0, y=y_0} $$

In our case the point is $P:(-1,1)$,

$$ y - 1 = \blue{\left( \frac{ 1-2y }{ 3 y^2 + 2 x }\right)}_{x=-1, y=1} (x +1),$$

$$ y - 1 = \blue{-1} (x +1),$$

$$ y - 1 = -x - 1,$$

$$ \red{\therefore y = -x }$$**2) **For the second question we have the following Initial Value Problem,

$$ \begin{cases} \frac{d P}{d y} + A y P = 0 \\ P(0)=1 \end{cases}$$

In this case, we have a Linear Differential Equation of the form,

$$ \frac{d P}{d y} + \blue{T(y)} P = 0 $$

so we need to consider an integrating factor given by

$$ \mu(y) = e^{\int \blue{T(y)} dy} = e^{\int \blue{A y} dy}= e^{\frac{A}{2} y^2}.$$

Then, we multiply the integrating factor to both sides of the Differential Equation,

$$ e^{\frac{A}{2} y^2}\left(\frac{d P}{d y} + A y P\right) = 0 $$

$$\frac{d}{dy}\left[ e^{\frac{A}{2} y^2} P(y) \right] = 0 $$

Then, we integrate to both sides

$$ e^{\frac{A}{2} y^2} P(y) = \green{K }$$

$$ P(y) = \green{K }e^{- \frac{A}{2} y^2},$$

where $\green{K}$ is a constant value.

Finally, we need to evaluate the function at the initial point $P(0)=1$,

$$ P(0) = \green{K }e^{- \frac{A}{2} 0^2} = \green{K}= 1$$

$$\red{\therefore P(y) =e^{- \frac{A}{2} y^2}}$$

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