A variant on the 1 - sequence, described in section 7.3.3 on page 148, is the sequence: 90(x) - - 90(y). Show that,

A variant on the 1 -  sequence, described in section 7.3.3 on page 148, is the sequence:
90°(x) - τ - 90°(y).
Show that, for a one-spin system, this sequence gives rise to trans-verse magnetization which varies as cos (Ωτ).
Hence show that there is a null in the excitation at Ω = π/(2τ); give the position of this null in frequency units (Hz). At what offsets is the excitation a maximum?