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Given that 
x=3y^(4)+1, find 
(d)/(dy)(5y^(2)-4cos x) in terms of only 
y.
Answer:

Given that x=3y4+1 x=3 y^{4}+1 , find ddy(5y24cosx) \frac{d}{d y}\left(5 y^{2}-4 \cos x\right) in terms of only y y .\newlineAnswer:

Full solution

Q. Given that x=3y4+1 x=3 y^{4}+1 , find ddy(5y24cosx) \frac{d}{d y}\left(5 y^{2}-4 \cos x\right) in terms of only y y .\newlineAnswer:
  1. Apply Chain Rule: To find the derivative of the function 5y24cos(x)5y^2 - 4\cos(x) with respect to yy, we need to apply the chain rule because xx is a function of yy. The chain rule states that the derivative of a composite function is the derivative of the outer function times the derivative of the inner function.
  2. Find Outer Function Derivative: First, let's find the derivative of the outer function 5y24cos(x)5y^2 - 4\cos(x) with respect to xx, which we will denote as f(x)f'(x). The derivative of 5y25y^2 with respect to xx is 00 since yy is treated as a constant with respect to xx. The derivative of 4cos(x)-4\cos(x) with respect to xx is xx00. So, xx11.
  3. Find Inner Function Derivative: Now, we need to find the derivative of the inner function xx with respect to yy, which we will denote as dxdy\frac{dx}{dy}. Given that x=3y4+1x = 3y^4 + 1, the derivative of xx with respect to yy is dxdy=ddy(3y4+1)=12y3\frac{dx}{dy} = \frac{d}{dy}(3y^4 + 1) = 12y^3.
  4. Apply Chain Rule Again: Using the chain rule, the derivative of the function 5y24cos(x)5y^2 - 4\cos(x) with respect to yy is the product of f(x)f'(x) and dxdy\frac{dx}{dy}. Therefore, ddy(5y24cos(x))=f(x)dxdy=4sin(x)12y3\frac{d}{dy}(5y^2 - 4\cos(x)) = f'(x) \cdot \frac{dx}{dy} = 4\sin(x) \cdot 12y^3.
  5. Express Derivative in Terms of yy: We need to express the derivative in terms of yy only. Since x=3y4+1x = 3y^4 + 1, we can substitute this into the sine function. So, sin(x)\sin(x) becomes sin(3y4+1)\sin(3y^4 + 1).
  6. Write Final Expression: Now, we can write the final expression for the derivative in terms of yy: ddy(5y24cos(x))=4sin(3y4+1)×12y3=48y3sin(3y4+1)\frac{d}{dy}(5y^2 - 4\cos(x)) = 4\sin(3y^4 + 1) \times 12y^3 = 48y^3\sin(3y^4 + 1).

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