13)
let assume # denotes angle signs
now
as sum of 3 angels in triangle is 180
#2 +#3 +#4 = 180
also
#1 + #4 = 180
so equating both equation
#2 +#3 + #4 = #1 + #4
#4 will be cancelled from both side then
#2 + #3 = #1
proved
14)
ratio is 1:3:5
so Alma adsed it wrong. it should be 9x in 2nd line.
correction
x + 3x + 5x = 180
9x = 180
x = 180/9 = 20
so 1st angle = x = 20
2nd angle = 3x = 3×20=60
3rd angel = 5x = 5×20 = 100