cos2@ = (1-tan2@) / (1+tan2@)
RHS = [1-(sin2@/cos2@)] / [1+(sin2@/cos2)] ----- express tan as sin/cos
= [(cos2@-sin2@)/cos2@] / [(cos2@+sin2@)/cos2@] ----- treat numerator fraction as one and create a common denominator of cos2@, do this for the denominator fraction too
= cos2@ - sin2@...