tolong bantu pakai cara ya....

a + c = 12 => c = 12 - a
b + c = 13 => b + 12 - a = 13 => b = a + 1
Aturan kosinus
a^2 = b^2 + c^2 - 2bc cos A
a^2 = (a + 1)^2 + (12 - a)^2 - 2 (a + 1)(12 - a) cos 60°
a^2 = a^2 + 2a + 1 + 144 - 24a + a^2 - 2 (-a^2 + 11a + 12) . 1/2
0 = a^2 - 22a + 145 - (-a^2 + 11a + 12)
0 = a^2 - 22a + 145 + a^2 - 11a - 12
0 = 2a^2 - 33a + 133
0 = (2a - 19)(a - 7)
a = 19/2 = 9,5 cm atau a = 7 cm