tolong bantu jawab yang nmor 4

4. a.
r = akar16 = 4
r baru = 2×4 = 8 cm

x^2 + y^2 = 8^2
x^2 + y^2 = 64

b.
r = akar(25/4) = 5/2
d = 2×5/2 = 5 cm

d baru = 2×5 = 10 cm
r baru = 10/2 = 5 cm

x^2 + y^2 = 5^2
x^2 + y^2 = 25

c.
menyinggung x=6 ==> r = 6 cm
x^2 + y^2 = 6^2
x^2 + y^2 = 36

d.
L = 81 phi
phi r^2 = 81 phi
r^2 = 81
r = akar81
r = 9 cm

x^2 + y^2 = 9^2
x^2 + y^2 = 81