Wednesday, 9 October 2019

algebra precalculus - Is this an incorrect proof of cot(x)+tan(x)=csc(x)sec(x)?

If you input the trig identity:
cot(x)+tan(x)=csc(x)sec(x)


Into WolframAlpha, it gives the following proof:



Expand into basic trigonometric parts:
cos(x)sin(x)+sin(x)cos(x)?=1sin(x)cos(x)


Put over a common denominator:




cos2(x)+sin2(x)cos(x)sin(x)?=1sin(x)cos(x)



Use the Pythagorean identity cos2(x)+sin2(x)=1:



1sin(x)cos(x)?=1sin(x)cos(x)



And finally simplify into



1?=1




The left and right side are identical, so the identity has been verified.



However, I take some issue with this. All this is doing is manipulating a statement that we don't know the veracity of into a true statement. And I've learned that any false statement can prove any true statement, so if this identity was wrong you could also reduce it to a true statement.



Obviously, this proof can be easily adapted into a proof by simply manipulating one side into the other, but:



Is this proof correct on its own? And can the steps WolframAlpha takes be justified, or is it completely wrong?

No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find limh0sin(ha)h without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...