maths about basic properties of circles

9.(a)
MB = AM = 3 cm (圓心對弦的垂直線平分該弦)
AB = AM + MB = 3 cm + 3 cm = 6 cm

DE = 6 cm

OE 垂直 DC (由圓心到弦的中線垂直該弦)
ΔOAM:
OD2 = OE2 + DE2 (畢氏定理)
OD2 = (4.5 cm)2 + (6 cm)2
OD2 = 56.25 cm2
半徑OD = 7.5 cm

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9.(b)
在直角 ΔOAM 中:
OA = OD = 7.5 cm (同圓半徑)
OM2 = OA2 - AM2 (畢氏定理)
OM2 = (7.5 cm)2 - (3 cm)2
OM2 = 47.25 cm2
OM = 6.9 cm
=

人地條問題係英文，Uncle Michael可否轉返做英文呢？