Panjang diagonal diagonal persegi (3x + 1) cm dan (x + 7) cm. Luas daerah persegi tersebut adalah...

3x + 1 = x + 7
3x - x = 7 - 1
2x = 6
x = 3

diagonal persegi = x + 7
= 3 + 7 = 10
sisi persegi = 1/2 × diagonal × diagonal
= 1/2 × 10 × 10
= 50 cm^2