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EXERCISE 1.2 · Q40

Q.Find the derivative of the function y=f(x)y=f(x) using the derivative of the inverse function x=f−1(y)x=f^{-1}(y): y=x−23y=\sqrt[3]{x-2}

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Since y=x−23y=\sqrt[3]{x-2}, cube both sides to get y3=x−2y^3=x-2, i.e. x=y3+2x=y^3+2 (the inverse relation, x=f−1(y)x=f^{-1}(y)). Differentiate xx w.r.t. yy: dxdy=3y2\dfrac{dx}{dy}=3y^2. Using dydx=1dx/dy\dfrac{dy}{dx}=\dfrac{1}{dx/dy}, we get dydx=13y2\dfrac{dy}{dx}=\dfrac{1}{3y^2}. Substituting back y=x−23y=\sqrt[3]{x-2}, so y2=(x−2)2/3=(x−2)23y^2=(x-2)^{2/3}=\sqrt[3]{(x-2)^2}, giving dydx=13(x−2)23\dfrac{dy}{dx}=\dfrac{1}{3\sqrt[3]{(x-2)^2}}.

✓Final answer

dydx=13(x−2)23\dfrac{dy}{dx}=\dfrac{1}{3\sqrt[3]{(x-2)^2}}

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