Question

A+B+C=180°=cos2A+cos2B+cos2C=

Answer 1

=cos2(A+B+C)
=cos2(180°) ________{A+B+C=180°}
=cos360°
=1 ________{cos360°=1}

Answer 2

=cos2A+cos2B+cos2C
=cos2(A+B+C)
=cos2(180°) ________{A+B+C=180°}
=cos360°
=1 ________{cos360°=1}