# Partial Derivatives Question…need some guidance.

Six Sigma – iSixSigma Forums Old Forums General Partial Derivatives Question…need some guidance.

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Dona
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I am given the task to compute the STD deviation of Y= 2*X1+3*X2^2, with X1=2, X1=3, std dev X1 = 0.1, std dev X2 = 0.2. I must use the partial dericatives method to get STD dev of Y. Here is my logic…I am not asking for a solution just some feed back on where I may be going wroing with this:
dY/dX1 = 2+3*X2^2 and dY/dX2 = 2*X1+6*X2 substituting in X1 and X2 I get 29 and 22 respectively.
Now if I want to find STD Dev of Y I need to use the following eqn:
STD dev Y = SQRT {(29*0.1)^2 + (22*0.2)^2)} this yields 5.26
Unfortunately 5.26 does not even come close to the choices I have for an answer. I have looked and looked and again and am completely stumped on this one. I would appreciate any direction you could give me.

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