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How do I input this kind of word problem into Wolfram Alpha Pro?

Posted 10 years ago
 The current i (in ?A) in a DVD player circuit is given by the equation i = 4.0 - 7.0t where t is time (in ?s) and 0 ? t ? 30 ?s. If q(subscript 0) = 0 C, excluding the initial charge, for what value of t is q = 0 C?
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Posted 10 years ago
Posted 10 years ago
 I think there is something missing. The result should be 0 C at t = 11 ?s.
Posted 10 years ago
 If you draw a graph of the current then I think from your description that will be a straight line with y intercept at 4 and a slope of -7.From t=0 to t=4/7 there will be current (charge) flowing into the circuit.To find where the cumulative charge is zero the area in the triangle above the x axis and below the line must equal the area in the triangle below the x axis and above the line. It seems that will happen when x=2*4/7.The WolframAlpha plot of the integral shows two orange points where the charge is zero, one when t=0 and the other when t=8/7 or 1.1428 microseconds which seems to match this description.Is there any chance there is something in the description of the problem that needs to be changed? Or did I misunderstand something?
Posted 10 years ago
 Let me double check the problem just to make sure the entry I made was correct.