. . , - . .
.
128π 3 ?
.
V = πr2 h
,
πr2 h = 128π
r2 h = 128
h = 128 / r2
. , :
S = 2πr2 + 2πrh
πr2 - ( )
2πr -
S = 2πr2 + 2πrh
S = 2πr2 + 2πr * 128 / r2
S = 2πr2 + 256π / r
, . .
f(r) = 2πr2 + 256π / r
. :
f '(r) = 4πr - 256π / r2
, f '(r) .
4πr - 256π / r2 = 0
4πr (1 - 64/r) = 0
4πr = 0 1 - 64/r = 0
r = 0 ,
1 - 64/r = 0
r = 64
h = 128 / r2
h = 128 / 4096
h = 0.03125 1/32
: h = 1/32 , r =64
. ( ). , - .
48 3. , , .
.
V = hr2
(V = 1/3hr2) "" , , , :
V = 1/3 h/2 r2
, , 48, hr2 = 48. hr2 . :
V = 1/3 * 48 / 2
V = 8 3.
: 8 3.
5, 12. .
.
. sqrt(), sqrt - , - .
:
S1 = rl - , r - , l -
S2 = r2 - ,
|
|
,
S = S1 + S2
S1 = rl, . , ,
l2 = h2 + r2
S1 = r * sqrt(h2 + r2)
S1 + S2 = r(h2 + r2) + r2 = * 12 * sqrt(25 + 144) + 144 * = * 12 * 13 + 144 * = 156 + 144 = 300 ≈ 942,48
: 300 ≈ 942,48 2.
. ( , ). , - .
.
36π 2, 10 .
.
.
, .
S = πR2
36π = πR2
R2 = 36
R = 6
:
S = πRl
R -
l -
S = π * 6 * 10 = 60π
: 60π 2.
. ( , ). , - .
.
, .
, .
.
, :
, , , , , . . :
h = 2R
V = πR2h = πR22R = 2πR3
V = 4/3πR3
V = 1/3πR2h = 1/3πR22R = 2/3πR3
, :
V / V = 2/3πR3 / 4/3πR3 = 2/3 / 4/3 = 1/2
:
V / V = 2/3πR3 / 2πR3 = 2/3 / 2 = 1/3
: 1/2 1/3
.
27. 2:1 . , . .
.
, AOB COD - . 2:3.
, .
V = 1/3πR2h = 27 ( )
V. = 1/3π(2/3R)2(2/3h)
V. = 1/3π 4/9 R2 2/3 h
V. = 8/27 *1/3π R2 h
, 1/3π R2 h= 27 (. ),
V. = 8/27 * 27 = 8
: 8
(2)
. ( , ). , - .
12 . 30 . .
.
:
, , .
|
|
h / OB = sin 30
h = OB sin 30
h = 12 sin 30
30 .
h = 12 * 1/2
h = 6
R / OB = cos 30
R = OB cos 30
R = 12 cos 30
30 .
R = 12 √3/2
R = 6√3
:
V = 1/3π (6√3)2 * 6
V = 216π
: 216π 2.