請問2兀r x4+2x兀x r x r=90兀等於多少?

2兀r x4+2x兀x r x r =90兀

8r+2r^2=90

r^2+4r-45=0

(r-5)(r+9)=0

所以,r=5或r=-9

(2pi r)(4)+(2pi r^2) = 90pi
8pi r+2pi r^2 = 90pi
pi(8r+2r^2) = 90pi
2r^2+8r = 90
2r^2+8r-90 = 0
(2r+18)(r-5) = 0
r = -9 or 5

2兀r x4+2x兀x r x r =90兀

8兀r+2兀r^2 =90兀

2兀r(4+r)=90兀

4+r=90兀÷2兀r

4+r=45÷r

r=5

2007-05-25 20:19:00 補充：
中間冇打4+r=45÷r4r+r×r - 45=0 r^2+9r-5r-45=0r(r+9)-5(r+9)=0(r-5)(r+9)=0r=5 or -9

我計到 0.446031029......