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Chapter 6 Class 12 Application of Derivatives
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Example 22 - Chapter 6 Class 12 Application of Derivatives

Last updated at May 29, 2023 by

Example 34 - Find two numbers whose sum is 15, sum of squares

Example 34 - Chapter 6 Class 12 Application of Derivatives - Part 2
Example 34 - Chapter 6 Class 12 Application of Derivatives - Part 3 Example 34 - Chapter 6 Class 12 Application of Derivatives - Part 4

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Example 22 Find two positive numbers whose sum is 15 and the sum of whose squares is minimum. Let first number be 𝒙 Since Sum of two positive numbers is 15 𝑥+ 2nd number = 15 2nd number = 15 – 𝒙 Let S(𝑥) be the sum of the squares of the numbers S(𝑥)= (1st number)2 + (2nd number) 2 S(𝒙)=𝒙^𝟐+(𝟏𝟓−𝒙)^𝟐 We need to minimize S(𝒙) Finding S’(𝒙) S’(𝑥)=𝑑(𝑥^2+ (15 − 𝑥)^2 )/𝑑𝑥 =𝑑(𝑥^2 )/𝑑𝑥+(𝑑(15 − 𝑥)^2)/𝑑𝑥 = 2𝑥+ 2(15−𝑥)(−1) = 2𝑥− 2(15−𝑥) = 2𝑥−30+2𝑥 = 4𝒙−𝟑𝟎 Putting S’(𝒙)=𝟎 4𝑥−30=0 4𝑥=30 𝑥=30/4 𝒙=𝟏𝟓/𝟐 Finding S’’(𝒙) S’’(𝑥)=𝑑(4𝑥 − 30)/𝑑𝑥 = 4 Since S’’(𝒙)>𝟎 at 𝑥=15/2 ∴ 𝑥=15/2 is local minima Thus, S(𝑥) is Minimum at 𝑥=15/2 Hence, 1st number = 𝑥=𝟏𝟓/𝟐 2nd number = 15−𝑥=15−15/2=𝟏𝟓/𝟐

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Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 13 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.