jagguy
asked on
finding x intercepts of sin graph
Hi,
I am confused about finding more than 1 x intercepts from a sin graph.
Take 2cos (2x-1)=0 for x intecept
cos2x=.5=60 deg
x=30deg is 1 intercept but what about 3 others ?
I am confused about finding more than 1 x intercepts from a sin graph.
Take 2cos (2x-1)=0 for x intecept
cos2x=.5=60 deg
x=30deg is 1 intercept but what about 3 others ?
But what is your function. f(x) = 2*cos( 2*x -1) ???
The first intercept for that is 2*x -1 = 90 ==> x = 91/2 = 45.5
The first intercept for that is 2*x -1 = 90 ==> x = 91/2 = 45.5
ASKER
sorry you will need to explain this more. my function is (x) = 2*cos( 2*x) -1
the first intercept is simply 30 deg, and to find the other intercepts what mathematics are we doing ?
if sqrt(2) *sin( x) + 1 there are only 2 intecepts in firs 2 pie. I can find one OK but the 2nd ?
the first intercept is simply 30 deg, and to find the other intercepts what mathematics are we doing ?
if sqrt(2) *sin( x) + 1 there are only 2 intecepts in firs 2 pie. I can find one OK but the 2nd ?
Cosine is 0 at ... -90, 90, 270, 450..... , 90+n*180,... where n is any integer
2cos (2x-1)=0
gives
2x-1 = 90+n*180 n any integer
x= (91 + n*180)/2
x =45.5 + n*90
Different values of n give different solutions,
for example n= -2, -1, 0, 1, 2.. we get
x = -134.5, -44.5, 45.5, 135.5, 225.5
2cos (2x-1)=0
gives
2x-1 = 90+n*180 n any integer
x= (91 + n*180)/2
x =45.5 + n*90
Different values of n give different solutions,
for example n= -2, -1, 0, 1, 2.. we get
x = -134.5, -44.5, 45.5, 135.5, 225.5
2*cos( 2*x) -1 = 0 is where it intercepts.
So 2*cos( 2*x) = 1
cos( 2*x) = 1/2
So you know that 30 degrees is the first intercept. The others will be anywhere else cos(2*x) = 1/2
Where does cos = 1/2? In one cycle, there are two places, then every intercept is 360n + one of them (where n is any integer).
For example, cos(60) = 1/2, cos(120) = 1/2 so cos (360*4 + 60) = 1/2 and cos (360*4 + 120) = 1/2
Of course you have 2x inside the cos, so you need to do some more math there.
So 2*cos( 2*x) = 1
cos( 2*x) = 1/2
So you know that 30 degrees is the first intercept. The others will be anywhere else cos(2*x) = 1/2
Where does cos = 1/2? In one cycle, there are two places, then every intercept is 360n + one of them (where n is any integer).
For example, cos(60) = 1/2, cos(120) = 1/2 so cos (360*4 + 60) = 1/2 and cos (360*4 + 120) = 1/2
Of course you have 2x inside the cos, so you need to do some more math there.
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Yeah. Oops. It's 60 and -60, not 60 and 120. Replace everywhere I said 120 with either -60 or 300 whichever you prefer.
..and a whoops for me 360 - 2x = 60 => x = 210 not 150
No. 150 was right.
TommySzalapski. Ty, I think I am loosing my marbles today.
all this about every 90 degrees gets to the bottom on the original question
BUT there is a big difficulty with finding the first intercept.
-
"cos2x=.5=60 deg
x=30deg is 1 intercept but what about 3 others ? "
What is this all about. I am lost
-
" 2*x -1 = 90"
you are mixing degrees and radians
stick to radians
BUT there is a big difficulty with finding the first intercept.
-
"cos2x=.5=60 deg
x=30deg is 1 intercept but what about 3 others ? "
What is this all about. I am lost
-
" 2*x -1 = 90"
you are mixing degrees and radians
stick to radians
Since you have a 2*x term, you will have intercepts every 90 degrees.