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## Topic review (newest first)

krisper
2006-01-14 22:21:22

kempos you are correct
It should be:

cos144 + cos72 = 2cos(144+72)/2*cos(144-72)/2=2cos108*cos36=
2cos(180-72)*cos36=-2cos72*cos36=(-cos72*2cos36*sin36)/sin36=
(-cos72*sin72)/sin36=(-1/2)*(sin144)/sin36=(-1/2)*(sin144)/(sin(180-36))=
(-1/2)(sin144)/(sin144)=-1/2

RickyOswaldIOW
2006-01-14 01:03:26

y? cos
TEHE

kempos
2006-01-14 01:01:55

cos144 + cos72 = 2cos(144+72)/2*cos(144-72)/2=2cos108*cos36=
!!!!!!!!!!2cos(180-108)!!!!!!!!!!!*cos36=-2cos72*cos36=(-cos72*2cos36*sin36)/sin36=
(-cos72*sin72)/sin36=(-1/2)*(sin144)/sin36=(-1/2)*(sin144)/(sin(180-36))=
(-1/2)(sin144)/(sin144)=-1/2

it should be 2cos(180-72)!!!

MathsIsFun
2006-01-13 22:29:54

Well done krisper, maybe you could tell us about yourself in Introductions

RauLiTo
2006-01-13 22:24:46

sirry rickyoswaldiow i didn't know that it was night
krisper thank you alot alot man

krisper
2006-01-13 21:33:31

cos144 + cos72 = 2cos(144+72)/2*cos(144-72)/2=2cos108*cos36=
2cos(180-72)*cos36=-2cos72*cos36=(-cos72*2cos36*sin36)/sin36=
(-cos72*sin72)/sin36=(-1/2)*(sin144)/sin36=(-1/2)*(sin144)/(sin(180-36))=
(-1/2)(sin144)/(sin144)=-1/2

RickyOswaldIOW
2006-01-13 21:18:08

There are a good few people on this board who will help you, but you must have a little patience.  They are probably in bed

RauLiTo
2006-01-13 19:17:58

no one can help me guys ?

RauLiTo
2006-01-13 06:13:17

how you doing ?

can you prove that :

cos 144 + cos 72 = - 1 / 2 ?

waiting for the answer like hell !