Math Problem Statement

: Cho hàm ẩn y = y(x) xác định bởi x 3 + y 3 − 6xy = 1. (a) Tìm y ′ (0). (b) Tìm y”(0).

Solution

Given the implicitly defined function y=y(x)y = y(x) by the equation

x3+y36xy=1x^3 + y^3 - 6xy = 1

we'll calculate y(0)y'(0) and y(0)y''(0).

(a) Find y(0)y'(0)

To find y(x)y'(x), we’ll implicitly differentiate both sides of the equation with respect to xx:

  1. Differentiate x3x^3: ddx(x3)=3x2\frac{d}{dx}(x^3) = 3x^2

  2. Differentiate y3y^3 using the chain rule: ddx(y3)=3y2y\frac{d}{dx}(y^3) = 3y^2 \cdot y'

  3. Differentiate 6xy-6xy using the product rule: ddx(6xy)=6(y+xy)\frac{d}{dx}(-6xy) = -6(y + x y')

Differentiating the entire equation x3+y36xy=1x^3 + y^3 - 6xy = 1 with respect to xx:

3x2+3y2y6(y+xy)=03x^2 + 3y^2 y' - 6(y + x y') = 0

Now, rearrange terms to solve for yy':

3y2y6xy=6y3x23y^2 y' - 6x y' = 6y - 3x^2

y(3y26x)=6y3x2y'(3y^2 - 6x) = 6y - 3x^2

y=6y3x23y26xy' = \frac{6y - 3x^2}{3y^2 - 6x}

To find y(0)y'(0), we substitute x=0x = 0 into the equation. First, we need to determine y(0)y(0).

Step 1: Find y(0)y(0)

Substitute x=0x = 0 in the original equation:

03+y360y=10^3 + y^3 - 6 \cdot 0 \cdot y = 1

y3=1y=1y^3 = 1 \Rightarrow y = 1

So, y(0)=1y(0) = 1.

Step 2: Substitute x=0x = 0 and y=1y = 1 into yy'

y(0)=6130231260=63=2y'(0) = \frac{6 \cdot 1 - 3 \cdot 0^2}{3 \cdot 1^2 - 6 \cdot 0} = \frac{6}{3} = 2

Thus,

y(0)=2y'(0) = 2

(b) Find y(0)y''(0)

To find y(0)y''(0), we need to differentiate yy' with respect to xx and evaluate it at x=0x = 0.

Given

y=6y3x23y26xy' = \frac{6y - 3x^2}{3y^2 - 6x}

Differentiating both the numerator and the denominator with respect to xx and applying the quotient rule will yield yy''. This calculation involves applying implicit differentiation again to account for y=y(x)y = y(x) in both terms.

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Math Problem Analysis

Mathematical Concepts

Implicit Differentiation
Calculus
Derivatives

Formulas

Implicit differentiation formula for derivatives
Quotient Rule for differentiation: (f/g)' = (f'g - fg') / g^2

Theorems

Chain Rule
Product Rule

Suitable Grade Level

Grade 11-12 or Early College