Question 1192245
duplicate
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Picking an angle and checking that the formula is correct does NOT necessarily mean it is correct for all angles.
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RE:  the formula:
If it is not correct for any angle tested, it is not valid.
It it is correct for any angle(s), it might be valid, might not be.
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As an example:
Prove sin(x) = cos(x)
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Let x = 45 degs
{{{sqrt(2)/2 = sqrt(2)/2}}}
QED
sin(x) <> cos(x) in general.
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It's possible the other tutor knew that formula from memory, but not likely.
He probably looked it up, as you could have done.
There are WAAAAYYYY too many formulas for anyone to remember, IMO.