ada yg bisa nomer 19,20,21? tolong dong hehe

19. cos C = AC² + BC² - AB² / ( 2)(AC)(BC)
cos C = (4²+4² - (4√2)²) / (2(4)(4) = 0/32
cos C = 0

20.
Tan 60 = AC/AB
AC = AB.  tan 60
AC = 4 (√3)
AC  = 4√3 cm

21.
a = 4
b = 6
c = 7
S= 1/2 ( a+b+c) =1/2 (4+6+7)
S= 8,5

L = √ [s(s-a)(s-b)(s-c)]
L = √(8,5)(4,5)(2,5)(1)
L = √95,625
L = 9,78