Proving 1 + tan^2X = 1 / cos^2X for 0 < X < 90 in a Right Triangle

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SUMMARY

The discussion focuses on proving the trigonometric identity 1 + tan²X = 1 / cos²X for angles X in the range 0 < X < 90 degrees using a right triangle. Participants clarify that tanX can be expressed as the ratio of the opposite side (b) to the adjacent side (a), while cosX is the ratio of the adjacent side (a) to the hypotenuse (c). By substituting these definitions into the original equation and applying the Pythagorean theorem, the proof becomes evident.

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Homework Statement



Use the given triangle to prove that for 0 < X <90, 1 + tan^2X = 1 / cos^2X

(the given triangle is right angled with angle X marked. The hypotenuse is labeled c, adjacent angle is labeled a and the opposite angle is labeled b)

The Attempt at a Solution



I have no idea where to begin,
tanX = b/a
cosX = a/c
 
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Welcome to PF!

Hi crazy_v! Welcome to PF! :wink:
crazy_v said:
Use the given triangle to prove that for 0 < X <90, 1 + tan^2X = 1 / cos^2X

tanX = b/a
cosX = a/c

But you're there

just put b/a and a/c into the original equation, and you have … ? :smile:
 
Since \cos \theta=\frac{a}{c} \Rightarrow \cos^2 \theta=\frac{a^2}{c^2}. Write c^2 in terms of a and b now, hint Pythagoras.
 
thanks guys

yeah that looks a lot more obvious now, thanks anyways
 

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