tolong ya no 18,19 pakai cara

sudut dalam segitiga

18.
<A + <B + <C = 180
70 + 180 -(x+40) + 180-(3x+30) = 180
70 + 180 -x -40 + 180 -3x - 30 = 180
360 - 4x = 180
-4x = -180
x = 45

<BCA = 180 - (3x +30)
<BCA = 180 - (135 +30)
<BCA = 180 - 165
<BCA = 15

19.
a+b +180-c = 360
-c = 360 - 180 -a- b
-c = 180 - a - b
c =  a + b - 180