JEE Advance - Mathematics (1985 - No. 30)
A ladder rests against a wall at an angle $$\alpha $$ to the horizintal. Its foot is pulled away from the wall through a distance $$a$$, so that it slides $$a$$ distance $$b$$ down the wall making an angle $$\beta $$ with the horizontal. Show that $$a = b\tan {1 \over 2}\left( {\alpha + \beta } \right)$$
The explanation is correct and sufficient.
The explanation is mostly correct, but misses a key step or detail.
The explanation is on the right track, but contains significant errors.
The explanation is irrelevant or completely incorrect.
There is no explanation provided.