# Thread: Finding delta y and dy?

1. ## Finding delta y and dy?

Hi, I submitted answers for my homework but got it wrong and I would like to know where I stumbled?

y = 3sqrt(x)

Find delta y when x = 5 and delta x = .2

for this I just figured that delta y would equal: f(5.2) - f(5) = .133

Find the differential dy when x = 5 and and dx = .2

I used the dy = fprime(x) dx formula to get .134

I got wrong results for both. It surprised me but is there something I haven't looked at? Thanks for clarification

2. This depends on what "delta y when x = 5 and delta x = .2" is. The things is that $\sqrt{5}-\sqrt{4.8}>\sqrt{5.2}-\sqrt{5}$, so if $\delta y$ is defined as the maximum of these things, then your answer is incorrect.

For the differential, though, I got the same answer as you did (up to thousandths).

3. Hmm, yes I do believe now that if I had answered with a 4 digit answer after the decimal it would have been right. Oh well haha, thanks for the info

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