The reason for that is because when dividing by cos^2(x) you are assuming that cos^2(x) is not equal to 0. So you need to also solve for the other case, namely that cos^2(x) does equal 0. Let me know if that helps :smile:

Reply 5

the bear

get everything on the left hand side... then you can factorize out the cosx...

Reply 6

username2866920

Original post by Mathlete18

Thanks I kinda understand but i dont know how to solve it.

Reply 7

username2866920

Original post by the bear

get everything on the left hand side... then you can factorize out the cosx...

Yh thats what I did but i got cosx(cosx+sinx) =0 idk how cosx=-sinx is gonna help

Reply 8

the bear

Original post by murat10q7

Yh thats what I did but i got cosx(cosx+sinx) =0 idk how cosx=-sinx is gonna help

there should be a - in the bracket, not a plus.

so cosx = 0 or cosx = sinx... now you should be thinking about tanx....

Reply 9

username2866920

Original post by the bear

there should be a - in the bracket, not a plus.

so cosx = 0 or cosx = sinx... now you should be thinking about tanx....

thanks i get it now cosx=0 tanx=1

Reply 10

quackers1999

Original post by the bear

there should be a - in the bracket, not a plus.

so cosx = 0 or cosx = sinx... now you should be thinking about tanx....

I think to solve this you to invade belgium

Reply 11

Mathlete18

Original post by murat10q7

Yh thats what I did but i got cosx(cosx+sinx) =0 idk how cosx=-sinx is gonna help

This is basically the same principle as solving for a factorised quadratic:

If you have a*b = 0 then either a=0 or b=0 or both equal 0.

In this case a = cosx and b = cosx - sinx

Therefore the solutions are any solutions to cosx = 0 or cosx - sinx = 0

This is why if you have (x-a)(x-b)=0 the solutions are x = a and x = b because those are the respective values that makes one of the brackets equal to 0.

Does that make sense?

(edited 7 years ago)

Reply 12

username2866920

Original post by Mathlete18

This is basically the same principle as solving for a factorised quadratic:

If you have a*b = 0 then either a=0 or b=0 or both equal 0.

In this case a = cosx and b = cosx + sinx

Therefore the solutions are any solutions to cosx = 0 or cosx + sinx = 0

This is why if you have (x-a)(x-b)=0 the solutions are x = a and x = b because those are the respective values that makes one of the brackets equal to 0.

Does that make sense?

Yep makes perfect sense thanks