Question

(a+b−c)² is equal to a) ( − + ) ² b) (− − + ) ² c) ( + + ) ² d) ( − − ) ²​

Answer 1

Answer: the formula is : a2+b2+c2+2ab+2bc+2ca

proof :

(a+b+c)2  can be written as (a+b+c)∗(a+b+c),

Therefore, (a+b+c)2=(a+b+c)∗(a+b+c)

=a(a+b+c)+b(a+b+c)+c(a+b+c)

=a2+ab+ca+ab+b2+bc+ca+bc+c2

=a2+b2+c2+ab+ab+bc+bc+ca+ca

=a2+b2+c2+2ab+2bc+2ca

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