數學唔識>< 3題 (20點)急急!

1.
(1/f) = (1/u) + (1/v)

When f=7 and v=14 :
(1/7) = (1/u) + (1/14)
1/u = (1/7) - (1/14)
1/u = (2/14) - (1/14)
1/u = 1/14
u = 14


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2.
D = b² - 4ac

When D = 136, b = -8 and a = 3 :
136 = (-8)² - 4(3)c
136 = 64 - 12c
12c = -72
c = -6


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3.
a)
No figure is given.
Suppose that L, w and V are the length, width and height of each rectangularblock.

V = 4Lwh

b)
When L = 55, w = 6 and V = 840 :
840 = 4(55)(6)h
h = 7/11

搞甚麼？答案 001 三題全錯，難道投票時沒看回答內容的嗎？

1. it is know that 1/f=1/u+1/v. if f=7 and v=14,find u.
Sub f and v.
1/7=1/u+1/14
3/14=1/u
14=3u
u=14/3
2.it is know that D=b^2-4ac.if D=136,b= -8 and a=3,find c
Sub D, b and a.
136=16-12c
120=-12c
c=-10
3.the solid on the right is composed of 4 identical rectangular blocks.
a) write down a formula for the volume V cm^3 of the solid.
(l)(w)(h)=V cm
b) If L=55,w=6 and V=840,find h
h=23/9