( ( ( The period ) ( of ( the wave ) ) ) ( ( ( ( is ( ( the reciprocal ) ( of ( its frequency ) ) ) ) and ) ( is ( expressed ( in seconds ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN period)) (PP (IN of) (NP (DT the) (NN wave)))) (VP (VP (VBZ is) (NP (NP (DT the) (NN reciprocal)) (PP (IN of) (NP (PRP$ its) (NN frequency))))) (CC and) (VP (VBZ is) (VP (VBN expressed) (PP (IN in) (NP (NNS seconds)))))) (. .)))
The period of the wave is the reciprocal of its frequency and is expressed in seconds.
(ROOT (S (NP (NP (DT The) (NN frequency)) (PP (IN of) (NP (DT a) (NN wave)))) (VP (VBZ is) (NP (NP (DT the) (NN reciprocal)) (PP (IN of) (NP (NP (DT the) (NN period)) (PP (IN of) (NP (DT the) (NN wave))))))) (. .)))