Question

Plz solve it with explanation

Answer 1

$in \: \: trangle \: \: \: abc \: \: \: \\ \\ \\ a + b + c = 180 \\ \\ \\ a + b = 180 - c \\ \\ \\ \\ divide \: both \: sides \: by \: 2 \\ \\ (a +b) \div 2 = 90 - c \div 2 \\ \\ \\ \\ \cos((a + b )\div 2) = \cos(90 - c \div 2) \\ \\ = \sin(c \div 2)$