Cecilia attempts to prove that 0=1. This was her proof:
−2016−3642−4⋅942−4⋅9+48142−2⋅4⋅29+(29)2(4−29)2(4−29)(4−29)+210=−20=25−45=52−5⋅9=52−5⋅9+481=52−5⋅9+(29)2=(5−29)2=(5−29)=(5−29)+21=1
Which is the first step where she made a mistake? Assume that the first line of the equation (−20=−20) is the first step.
Signs: a⋅b⋅c=(a×b×c).