Question

Help Please!!!!
supposed to find y coordinate plugging in x value into y=cos pix . first value is .4 but not sure if I just multiply .4 by -1 or multiply cos pi x .4?

Answer 1

\[\large{y = \cos(\pi*x)}\]

Right ?

Answer 2

that's what im confused on ?... because the problem tells me that the point (.5,0) lies on the curve y=cos pix and the next x coordinate is .4 but I'm not understanding if I multiply by -1 ?... or what cause I dont' understand how .5,0 is a point in the same graph

Answer 3

Sorry:

No don't multiply it by -1

Answer 4

See... cos(pi x) is not always equal to -1...

Instead for integral values of x:

\[\large{\cos(\pi x) = (-1)^x}\]

Answer 5

But since here x is not integer so it is better to just multiply x by pi and then find its cos to get the answer

Answer 6

ok. so actually I just take the -1 and raise it to the x value?

Answer 7

Yeah if x is an integer.... if it is not an integer then a big NO

Answer 8

Here x is 0.4 which is NOT an integer so you CANNOT use (-1)^x...

Answer 9

oh. so...
I just have to find it graphically ?

Answer 10

No ... you can use this method:

\[\large{\cos(\pi x) = \cos(\pi * 0.4) = \cos(0.4 \pi)}\]

Now you can use calculator to find out cos(0.4pi)

Answer 11

You can also find it graphically if you want but it would be a tad difficult

Answer 12

so that would give me .99975...

Answer 13

How ?

Answer 14

See:

\[\large{\cos(0.4\pi) = \cos(0.4*180)}\] (in degrees)

\[\large{= \cos(72)}\]

= 0.309016

Answer 15

ooooohh... ok. i'm completely forgot degrees.. whoops.:(

Answer 16

:)

Its okay

Answer 17

thank you vishweshrimali5 you have opened my mind. :) much thanks.

Answer 18

Your welcome :)