213. In an electric circuit, two resistors with resistances x
and y are connected in parallel. In this case, if r is the
combined resistance of these two resistors, then the
reciprocal of r is equal to the sum of the reciprocals of
x and y. What is r in terms of x and y ?
(A) xy
(B) x + y
(C)
1
x + y
(D)
xy
x + y
(E)
x + y
xy
主要是看不明白题，请会的指点指点~谢谢啦
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