Question

cos2x < 0, 0 13 years ago

Answer 1

0 is like cos(pi/2 + n*pi)

cos(2x) = cos(pi/2 + n*pi)
2x = pi/2 + n*pi
x = pi/4 + n*pi/2

so no since we know 0<x<180
x = pi/4
x = pi/4 + pi/2

now we have to check :
we take number smaller than pi/4 but larger than 0 for example pi/8
we get there cos(2*pi/8) > 0

we take number between pi/4 and pi/4 + pi/2 for example pi/3
we get there cos(2*pi/3) <0

we take number larger than pi/4 + pi/2 but smaller than pi for example 5pi/6
we get there cos(2*5pi/6) >0

so the answer is pi/4 < x < pi/4+pi/2

Answer 2

ohhh, cool :) Thank you soo muchh :D

Answer 3

yw.. if you think about it it is quite obvious
we know that cos(x) is negative between 90 and 270
if you divide it by two you get 45 and 135 :)

Answer 4

please close this question if done