Here's a problem not even my teacher could figure out.

Everyone knows thatcot(90 + y) = -tan y

So now we have toproveit.

cot(90 + y) = 1/(tan 90 + y)

LHS = {1 - (tan 90)(tan y)} / {(tan 90)(tan y}

But what the heck are we to do with tan 90???

And how will one prove this identity?