Question

solve cos3A+sinA-cosA=0,for 0 14 years ago

Answer 1

Hoot! You just asked your first question! Hang tight while I find people to answer it for you. You can thank people who give you good answers by clicking the 'Good Answer' button on the right!

Answer 2

really? i think you get a cubic equation if you try to solve this.

Answer 3

i read it as
\[\cos(3A)\] but perhaps it is
\[\cos^3(A)\] for the first term

Answer 4

read it as cos(3A)

Answer 5

then you are in trouble

Answer 6

haha

Answer 7

well you have to use a triple angle formula for cosine. then try to rewrite it in term of only one trig function, but i am fairly sure you will get a cubic, and solving will not be easy

Answer 8

haha thx anywayz but i believe there is a simpler way will get back to u i i got it

Answer 9

i'd love to see it. pi works. 0 works. if you just want an answer and see how ugly it is try this
http://www.wolframalpha.com/input/?i=cos%283A%29%2Bsin%28A%29-cos%28A%29%3D0

Answer 10

haha thx

Answer 11

let me know the "easy" way to get the weird looking ones

Answer 12

ok

Answer 13

hey if i think if we change cos3A to cos(2A+A) it might be easier to solve since we can use trigo addition formular

Answer 14

well cos3A - cos A = -2sin2AsinA

so we got -2sin2AsinA + sinA = 0

Answer 15

how can we proceed now

can we divide by sinA??
giving -2sin2A + 1 = 0
so sin2A = 0.5
2A = 30, 150, 390, 510

then A =15, 75, 195, 255

but 0 and 360 are roots also

Answer 16

is uppose you lose some roots by divivding by sinA

Answer 17

they are angles
heard of the ASTC method??

Answer 18

how did u get-2sin2Asin2A ??

Answer 19

ASTC no

Answer 20

by the 'sums and differences' formulae

there are four

Answer 21

trigo addition formular??

Answer 22

the one I used is

CosA - cos B = -2 sin(A+B)/2 * sin(A-B/2

Answer 23

read as 'cos - cos = - 2sin semi sum sin semi difference'

Answer 24

factor formular i see i see

Answer 25

i'll check some of the results

Answer 26

ok

Answer 27

soc45 + sin15 - cos 15 = 0

that was is right anyway

Answer 28

solved
A=0,90,180

Answer 29

those are the answered given?

Answer 30

urs correct mine wrong
had a careless mistake

Answer 31

no

Answer 32

thx

Answer 33

I had to use one of those formulae a liitle while ago otherwise I wouldn have remembered them They can be useful!!

OK u welcome.


Pls tick the good answer box to close the question

Answer 34

haha done thx once again

Answer 35

nice work jimmyrep! i got
\[\frac{\pi}{12}\] by guessing, but did not get the others