1.
to know that the circumference=2pi*r
2pi*r=50
pi*r=25
r=50/pi
so the area
=pi*(50/pi)^2
=795.774715
=796㎠,cor to 3 sig fig
2.
the area = pi*r^2
so
pi*r^2=42
r^2=42/pi
r=sqrt(42/pi)
so the circumference
=2pi*sqrt(42/pi)
=22.9736276
=23.0m2,cor to 3 sig fig
Chinese version
1)
設該項鍊所圍成之圓的半徑為r cm
2πr=50 (圓周界公式=2πr)
πr=25
r=25/π
所以該項鍊所圍成之圓的面積:
πr^2(圓面積公式=πr^2)
= 25*25/π
= 198.9cm^2 (準確至3位有效數字)
2)
設圓形草地的半徑為r cm
πr^2=42
r=3.66 (準確至3位有效數字)
所以圓形草地的籬笆的長度:
2πr
=2π (3.66)
= 23.0 cm (準確至3位有效數字)
english version1.
to know that the circumference=2pi*r
2pi*r=50
pi*r=25
r=50/pi
so the area
=pi*(50/pi)^2
=795.774715
=796㎠,cor to 3 sig fig
2.
the area = pi*r^2
so
pi*r^2=42
r^2=42/pi
r=sqrt(42/pi)
so the circumference
=2pi*sqrt(42/pi)
=22.9736276
=23.0m2,cor to 3 sig fig