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I have a problem that I can't solve. Please help me.

Here is it:

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Verify that the function y = -(cos x) ln (sec x + tan x) is an explicit solution of the differential equation y'' + y = tan x. Assume an appropriate interval I of definition.

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First of all, I don't understand the meaning of "Assume an appropriate interval I of definition".

Second, I tried to differnetiate y = -(cos x) ln (sec x + tan x) twice in order to get y'' and replace it in the function but I couldn't. It's realy difficult and terribly long. I think I'm doing it in the wrong way. Please help me in that.

Thanks.

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# Solution for the Differential Equation

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