Proof problem: (cosα- 1)? +sin? α=2-2cosα proves that both sides are equal. This is the first volume of mathematical problems in the basic module of technical secondary school.
(cosα- 1)? +sin? α
=cos? α-2cosα+ 1+sin? α
=(cos? α+sin? α)+ 1-2cosα
= 1+ 1-2cosα
=2-2cosα
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