F.2 MAths

1. Three circular gold coins have the same thickness and their radii are 2 cm,
3cm, and 4 cm respectively. The three gold coins are melted and recast into
one new gold coin. If the new gold coin is just as thickas each of the original
three gold coins, find the radius of the new gold coin, correct to 2 significant
figures.


Let the thickness of the gold coins be x cm.
The volume of the three original coins:
2²pi x+3²pi x+4²pi x
=4x pi+9x pi+16x pi
=29x pi

Therefore, the radius of the new gold coin:
√[(29x pi)/(x pi)]
=√29
=5.4(cor. to 2 sig. fig.)

2007-06-10 12:20:30 補充：
=29x pi該是=29x pi(cm³)=5.4(cor. to 2 sig. fig.)該是=5.4(cm)(cor. to 2 sig. fig.)