Ex 6.1,5

Transcript

Ex 6.1,5 A stone is dropped into a quiet lake and waves move in circles at the speed of 5 cm/s. At the instant when the radius of the circular wave is 8 cm, how fast is the enclosed area increasing? Let r be the radius of circle & A be the Area of circle When stone is dropped into a lake waves move in a circle at the speed of 5 cm/sec Thus, 𝑑𝑟﷮𝑑𝑡﷯ = 5cm/sec We need find how fast area increasing w. r. t time when radius is 8cm i.e. we need to find 𝑑𝐴﷮𝑑𝑡﷯ when r = 8 cm. We know that Area of circle = πr2 A = πr2 Different w. r. t time 𝑑𝐴﷮𝑑𝑡﷯ = 𝑑(𝜋𝑟2)﷮𝑑𝑡﷯ 𝑑𝐴﷮𝑑𝑡﷯ = π 𝑑(𝑟2)﷮𝑑𝑡﷯ 𝑑𝐴﷮𝑑𝑡﷯ = π 𝑑(𝑟2)﷮𝑑𝑡﷯ × 𝑑𝑟﷮𝑑𝑟﷯ 𝑑𝐴﷮𝑑𝑡﷯ = π 𝑑(𝑟2)﷮𝑑𝑟﷯ × 𝑑𝑟﷮𝑑𝑡﷯ 𝑑𝐴﷮𝑑𝑡﷯ = π × 2r. 𝑑𝑟﷮𝑑𝑡﷯ 𝑑𝐴﷮𝑑𝑡﷯ = 2πr . 𝒅𝒓﷮𝒅𝒕﷯ 𝑑𝐴﷮𝑑𝑡﷯ = 2πr . 5 𝑑𝐴﷮𝑑𝑡﷯ = 10πr When r = 8 cm 𝑑𝐴﷮𝑑𝑡﷯﷯﷮𝑟 = 8﷯ = 10 × π × 8 𝑑𝐴﷮𝑑𝑡﷯﷯﷮𝑟 = 8﷯ = 80π Since Area is in cm2 & time is in sec 𝑑𝐴﷮𝑑𝑡﷯﷯﷮𝑟 = 8﷯= 80π 𝑐𝑚2﷮𝑠𝑒𝑐﷯ 𝑑𝐴﷮𝑑𝑡﷯﷯﷮𝑟 = 8﷯= 80π cm2/sec Hence Area is increasing at the rate of 80π cm2/sec when r = 8 cm