Answer:
Step-by-step explanation:
We know that sin²Ф+cos²Ф=1
when you divide both sides by cos²Ф you get
sin²Ф/cos²Ф+cos²Ф/cos²Ф=1/cos²²Ф
sin²Ф/cos²Ф=tan²Ф
cos²Ф/cos²Ф=1
1/cos²Ф=sec²Ф
tan²Ф+1=sec²Ф
subtract tan²Ф on both sides
1=sec²Ф-tan²Ф
therefore sec²Ф-tan²Ф=1
