帮帮我解决ADD MATH

oceanheng

solve the simultaneous equation
V = 60 sin x

(4/5)V = 36 cos x -15

menglee90

回复 1# oceanheng


   这就是完整的问题？想确定下楼主有没有type错？

TALL

回复  oceanheng


   这就是完整的问题？想确定下楼主有没有type错？
menglee90 发表于 9-7-2011 10:17 AM

    对咯！lz有没有答案？

lw.loon

36cos(x-15)还是36cos(x)-15？

kaiyilim

solve the simultaneous equation
V = 60 sin x

(4/5)V = 36 cos x -15
oceanheng 发表于 8-7-2011 08:17 PM

V=60 sin x--------Eqn 1
(4/5)V= 36 cos x - 15----Eqn 2

Substitue Eqn 1 into Eqn 2

(4/5)(60sin x)= 36 cos x - 15

48sin x = 36 cos x - 15

(拿36 cos x to the other side)

48sin x / 36 cos x = -15

(48/36)(sin x/ cos x) = -15

(Note that sin x/ cos x= tan x. Refer to Form 5 Chap 5 Trigonometric Functions)

Therefore,
(48/36)(tan x) = -15

(tan x)= (-15)(36/48)

x= tan"(-15)(36/48)

(Note that " is -1 up the tan there.我没有calculator, paiseh~)
对吗？agak agak 做的， 对或错都请回复

menglee90

回复 5# kaiyilim


   48sin x = 36 cos x - 15

(拿36 cos x to the other side)

48sin x / 36 cos x = -15

应该是 48sin x - 36 cos x = -15 吧。。。

yisonhome

i assume your question is correct which is 4/5V = 36 cos x - 15

solution is


4/5 (60sin x) =  36 cos x - 15  ----------sub V= 60sin x

4 (12sin x) =  36 cos x - 15 -----------simplify 5 and 60

48 sin x = 36 cos x - 15

48 tan x = 36 - (15/cos x)  ------------divide cos x to each term)

48 tan x = 36 - 15 sec x -------------- 1/ cos x = sec x

16 tan x = 12 - 5 sec x --------------divide each term with 3

12 - 16 tan x = 5 sec x

(12 - 16 tan x)2 = (5 sec x) 2 -------square both side

144 - 384 tan x + 256 tan2 x = 25 sec 2 x

144 - 384 tan x + 256 tan2 x = 25 (1 + tan2 x )

144 - 384 tan x + 256 tan2 x = 25 + 25tan2 x

231 tan2 x - 384 tan x + 119 = 0  ---------using quadratic formula to solve

a = 231, b = - 384 , c = 119

x = (-b ± √ b2 - 4 ac) / 2a


after u sub into the formula, the answer should be

x = 1.250 and 0.412

kaiyilim

回复 6# menglee90

Sorry,...><
谢谢通知

TALL

i assume your question is correct which is 4/5V = 36 cos x - 15

solution is


4/5 (60sin x) =  ...
yisonhome 发表于 13-7-2011 06:56 PM

按照酱算的话

不是X=1.250，而是tanX=1.250。。还得inverse。。。

yisonhome

不好意思， 酱x = 51.35, and x = 22.39