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Rate of Change of Area of circle with respect to radius r at r = 6 cm

Ex 6.1,17 - Chapter 6 Class 12 Application of Derivatives - Part 2

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Ex 6.1, 17 The rate of change of the area of a circle with respect to its radius r at r = 6 cm is (A) 10๐œ‹ (B) 12๐œ‹ (C) 8๐œ‹ (D) 11๐œ‹Let r be the radius of circle & A be the area of circle We need to find Rate of change of Area of circle w.r.t to radius at ๐‘Ÿ = 6 i.e. we need to find ๐’…๐‘จ/๐’…๐’“ at ๐’“ = 6 We know that Area of circle = A = ๐œ‹ ๐‘Ÿ^2 Differentiating w.r.t ๐‘Ÿ ๐‘‘๐ด/๐‘‘๐‘Ÿ=๐‘‘(๐œ‹๐‘Ÿ^2 )/๐‘‘๐‘Ÿ ๐‘‘๐ด/๐‘‘๐‘Ÿ=๐œ‹ ๐‘‘(๐‘Ÿ^2 )/๐‘‘๐‘Ÿ ๐‘‘๐ด/๐‘‘๐‘Ÿ=๐œ‹ . 2r ๐’…๐‘จ/๐’…๐’“=๐Ÿ๐…๐’“ Putting ๐‘Ÿ = 6 ใ€–๐‘‘๐ด/๐‘‘๐‘Ÿโ”‚ใ€—_(๐‘Ÿ = 6" " )=2๐œ‹(6) ใ€–๐‘‘๐ด/๐‘‘๐‘Ÿโ”‚ใ€—_(๐‘Ÿ = 6" " )=๐Ÿ๐Ÿ๐… Hence, Correct answer is B

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