Find the surface area

- anonymous

Find the surface area

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- anonymous

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- anonymous

and this

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- dumbcow

hello
for the first one you need to use formula for SA of sphere and then plug in the radius
so
what is SA of a sphere?
what is the radius of our sphere?

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## More answers

- anonymous

so i have to divide 1.8 by 2

- dumbcow

correct because radius is half the diameter

- anonymous

i got 0.9

- dumbcow

now substitute that in for r in SA formula

- anonymous

ok i am doing it right now

- anonymous

i did the volume then i got 0.972pi

- dumbcow

correct for volume
i thought we were doing surface area

- anonymous

yea but my homework say to find both of them

- dumbcow

oh ok
what would SA be then

- anonymous

3.24pi

- dumbcow

good job
for next one do the same thing except at the end we will divide by 2 because its only half a sphere

- anonymous

what do you mean?

- dumbcow

it shows image of half sphere with radius of 12 right?
so find volume and SA using same formulas substituting 12 for r
but we only want half the volume and half the SA

- anonymous

i find the volume is 1152pi

- dumbcow

correct

- anonymous

SA is 432pi right?

- dumbcow

no i get 288pi
SA = 4pi*12^2 = 4pi*144 = 576pi
take half of 576
288

- anonymous

ok thank here the another question:
The shaded circle at right has area 40π cm2. Find the surface area of the sphere.

- dumbcow

i dont understand how are the circle and sphere related?

- anonymous

here the hint:The surface area of a sphere is how many times the area of a circle with the same radius?

- dumbcow

ok so they share the same radius
is the entire circle shaded?

- anonymous

here the picture

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- dumbcow

ok thanks
use area of circle formula to find radius, then plug that into SA formula
A = pi*r^2 = 40pi

- anonymous

i have to divide pi in both side

- dumbcow

yes

- anonymous

i find the raidus 6.32455532

- dumbcow

correct

- dumbcow

SA=4pi*(6.32)^2

- anonymous

159.7pi

- dumbcow

correct and you can round up to 160 since we rounded the radius

- anonymous

Find the volume of a sphere whose surface area is 64π cm2.

- dumbcow

first find radius
4pi*r^2 = 64pi

- anonymous

4

- dumbcow

good
now use that to find volume
V = 4/3pi*r^3

- anonymous

i got 256pi over 3

- dumbcow

correct

- anonymous

Find the surface area of a sphere whose volume is 288π cm3.

- dumbcow

same thing we just did except use volume formula to find radius
V = 4/3pi*r^3 = 288pi

- anonymous

what should i do with 4/3 pi?

- dumbcow

well first get rid of pi by dividing pi on both sides like last time
then we will divide by 4/3

- anonymous

x by 3/4 by both side

- dumbcow

correct

- anonymous

what about 288*3/4

- dumbcow

think of it as (288*3)/4

- anonymous

i got 6 as radius

- dumbcow

correct put that in SA formula

- anonymous

144pi

- dumbcow

correct

- anonymous

if the radius of the base of a hemisphere (which is bounded by a great circle) is r, what is the area of the great circle? What is the total surface area of the hemisphere, including the base? How do they compare?

- anonymous

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- dumbcow

you can do this one, its just using the formula for circle area and SA
the radius is just r

- anonymous

ok i will but i need to leave right but i tell you when i get home

- dumbcow

k

- anonymous

ok i'm back so my homewprk paper has Agreat cicrle=,Shemisphere= and SA Hemisphere=__xA of great circle

- dumbcow

yes lets fill in the gaps
A = area circle
SA = area circle + 1/2 SA of sphere

- anonymous

the area of that is pir^2

- dumbcow

good
A = pir^2

- anonymous

we know the radius is r so that mean..

- dumbcow

whats SA of sphere

- anonymous

4pir^2

- dumbcow

good
SA hemishere = pir^2 +(1/2)*4pir^2
add like terms

- anonymous

3pir^2

- dumbcow

good
and A = pir^2 right?
so SA = ___ * A

- anonymous

what the of Agreat cicrle= and Shemisphere=

- anonymous

i forgot to put answer

- dumbcow

A = pir^2
SA hemishere = pir^2 +(1/2)*4pir^2 add like terms

- anonymous

so that is the answer or i have to solve it

- dumbcow

no thats the answer, remember we dont know what r is

- anonymous

oh ok so SA = r * A

- dumbcow

well does that sense?
sub in what we know
SA = 3pir^2 and A=pir^2
3pir^2 = r*pir^2
is that true?

- anonymous

not really

- dumbcow

ok look at it this way
SA = 3pir^2
if i plug in A for pir^2
i get SA = 3A

- anonymous

oh now i got it
If Jose used 4 gallons of wood sealant to cover the hemispherical ceiling of his vaction home, how many gallons of wood sealant are needed to cover the floor?

- dumbcow

ok use what we just learned
SA = 3A = 4 gal + gal for the floor

- anonymous

rid of a and 5 gal

- dumbcow

sorry i was unclear there
we dont know num of gall for the floor, lets call it x

- anonymous

ok

- dumbcow

we know SA of top is 2*pir^2
SA = 2pir^2 = 2A

- dumbcow

2A = 4 gall

- anonymous

then divide by 2

- anonymous

A = 2 gal

- dumbcow

yes

- dumbcow

total = 3A
A = 2 gall
so total = 3*2 = 6 gallons

- anonymous

i got it thank you so much

- dumbcow

your welcome

- anonymous

if i need help for my homework can u help me

- dumbcow

yeah if you post it and im on sure

- anonymous

thank again ^_^ that help me a lot from you

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