數學f.2 5題><20點 幫下忙...好難...

Questions are not written clearly...


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2012-11-01 21:58:32 補充：
(5) Given s=ut+1/2 at^2

2012-11-01 21:59:09 補充：
文字版：

(1)
(7xy^2-21x^2 y)/(-42xy+14y^2 )
=(7xy(y-3x))/(14y(-3x+y))
=x/2

(2)
x^2/(3-3x)×(-3)/x÷x/(x-1)
=x^2/(3(1-x))×(-3)/x×(-(1-x)/x)
=x/(3(1-x))×(-3)/1×(-(1-x)/x)
=-x/(1-x)×-1×(1-x)/x
=-(-1)
=1

2012-11-01 21:59:18 補充：
(3)

4m/(m-2n)-8m/(2m+n)
=(4m(2m+n))/((m-2n)(2m+n))-8m(m-2n)/((2m+n)(m-2n) )
=(4m(2m+n)-8m(m-2n))/((m-2n)(2m+n))
=(8m^2+4mn-8m^2+16mn)/((m-2n)(2m+n))
=20mn/((m-2n)(2m+n))

2012-11-01 21:59:53 補充：
(4)

What is the subject to change?
r= st/(s+t) [s]
r= st/(s+t)
r(s+t)=st
rs+rt=st
st-rs=rt
s(t-r)=rt
s= rt/(t-r)


r= st/(s+t) [t]
r= st/(s+t)
r(s+t)=st
rs+rt=st
st-rt=st
t(s-r)=rs
t=rs/(s-r)

2012-11-01 22:00:08 補充：
(5)

Given s=ut+1/2 at^2

Change the subject of formula to u.

s=ut+1/2 at^2
ut=s- 1/2 at^2
u= (s-1/2 at^2)/t
u= (2s-at^2)/2t

2012-11-01 22:00:19 補充：
(b) If s = 580, t = 10 and a = 7, find the value of u.

u= (2s-at^2)/2t
u= (2(580)-7(〖10〗^2))/(2(10))
u= (1160-700)/20
u= 460/20
u=23

Please use bracket properly.

e.g. r = st/(s+t)