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Mathematics 65 Online
OpenStudy (anonymous):

use the definition of derivative to find f'(x) for f(x)=(3-5x)^1/2

OpenStudy (bahrom7893):

limit definition is f'(x) = lim as h -> 0 [f(x+h) - f(x)]/h

OpenStudy (bahrom7893):

plug that in: lim as h -> 0 {(3 - 5(x+h))^(1/2) - (3-5x)^(1/2)}/h

OpenStudy (bahrom7893):

now to get rid of square roots, use the identity a^2 - b^2 = (a-b)(a+b) here you have a - b, but are missing a + b. So multiply top and bottom by: (3 - 5(x+h))^(1/2) + (3-5x)^(1/2)

OpenStudy (bahrom7893):

lim as h -> 0 [ (3 - 5(x+h))^(1/2) - (3-5x)^(1/2) * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ]/[ h * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ]

OpenStudy (bahrom7893):

that is equal to the: lim as h -> 0 [ (3 - 5(x+h)) - (3-5x) ] / [ h * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ]

OpenStudy (bahrom7893):

simplify numerator: lim as h -> 0 of [ (3 - 5(x+h)) - (3-5x) ] / [ h * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ] = = lim as h -> 0 of [ 3 - 5x-5h - 3+5x) ] / [ h * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ] = = lim as h -> 0 of [-5h]/[ h * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ]

OpenStudy (bahrom7893):

Cancel out "h"s lim as h -> 0 of [-5h]/[ h * (3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ] = lim as h -> 0 of [-5]/[(3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ]

OpenStudy (bahrom7893):

Plug in h = 0: lim as h -> 0 of [-5]/[(3 - 5(x+h))^(1/2) + (3-5x)^(1/2) ] = [-5]/[(3 - 5(x)^(1/2) + (3-5x)^(1/2) ]

OpenStudy (bahrom7893):

As long as I didn't make any arithmetic/simplification errors, that is the answer, you can check it by taking the derivative directly. Please click on become a fan if I helped, I really want to get to the next level!! Thanks =)

OpenStudy (anonymous):

thanx for ur reply

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