nomor 12&13 pakai cara

12.  Lingkaran dalam
PQ = 12 cm
QR = 5 cm
PR² = PQ² + QR²
PR² = 12²+5² = 169
PR = √169 = 13

jari jari lingkaran dalam = r = 1/2 PR= 1/2(13) = 6,5
Keliling = 2πr = 2(3,14)(6,5) = 40,82

13.
Δ PQR sama sisi
s=12√3
L = 1/4 s² √3
L = 1/4 . (12√3)² √3
L = 1/4. 432 √3
L = 108√3

R = abc / 4L
R = (12√3)³ / 4(108 √3)
R = 5.184 √3/ 4(108) √3
R = 12