d/dx ( tanx) = (secx)^2
That's why!
d/dx？ (? tanx)？ =? (secx)^2
∫? (tanx)^2？ Advanced (short for deluxe)
=∫? 【(secx)^2？ - 1]? Advanced (short for deluxe)
=∫? (secx)^2dx？ -∫dx
=tanx？ -? x？ +? c