Homework proving and solving identities

Kellyanne Smith

New member
Hello I need help with the following question I have an idea on how to go on but for some reason, I am having issues making them the same
1)Prove that sin 2x + 2 sin^2(45 - x ) = sin^2 x + cos^2 x
(the 45 is in degrees)

Last edited:

Dr.Peterson

Elite Member
Hello I need help with the following question I have an idea on how to go on but for some reason, I am having issues making them the same
1)Prove that sin 2x + 2 sin^2(45 - x ) = sin^2 x + cos^2 x
(the 45 is in degrees)
Please show us what you've done, so we can nudge you in the right direction or correct errors. We need to see your "issues" in order to deal with them.

I assume you've used the angle-subtraction identity and the double-angle identity.

Kellyanne Smith

New member
yes i have i replaced the sin 2x with 2 sinx cos x and replaced the sin^2(45-x) with (sqrt 2/2 cos x - sqrt 2/2 sin x) all squared.
then I went along simplifying the expression I have rewritten the (sqrt 2/2 cos x - sqrt 2/2 sin x) all squared as a whole fraction (sqrt 2 cos x - sqrt 2 sin x / 2) then I raised what is in the bracket to the power of 2. and from there I began to doubt what I was doing

Jomo

Elite Member
You stopped just where you claim you are having trouble! Please show us your work so we can find your error. Do you know what the right hand side reduces to?

Jomo

Elite Member
Hello I need help with the following question I have an idea on how to go on but for some reason, I am having issues making them the same
1)Prove that sin 2x + 2 sin^2(45 - x ) = sin^2 x + cos^2 x
(the 45 is in degrees)
..and the x (in 45-x) is also in degrees. Best to say that the whole angle, 45-x, is in degrees.

Subhotosh Khan

Super Moderator
Staff member
yes i have i replaced the sin 2x with 2 sinx cos x and replaced the sin^2(45-x) with (sqrt 2/2 cos x - sqrt 2/2 sin x) all squared.
then I went along simplifying the expression I have rewritten the (sqrt 2/2 cos x - sqrt 2/2 sin x) all squared as a whole fraction (sqrt 2 cos x - sqrt 2 sin x / 2) then I raised what is in the bracket to the power of 2. and from there I began to doubt what I was doing
so you did the following:

sin 2x + 2 sin^2(45 - x )

= 2 * sin(x) * cos(x) + 2 * $$\displaystyle \left[\frac{\sqrt{2}}{2} * cos(x) - \frac{\sqrt{2}}{2} * sin(x) \right]^2$$

After that I, I would factor out the (√2)/2 and realize that [(√2)/2]2 = 1/2 and continue.....

Kellyanne Smith

New member
You stopped just where you claim you are having trouble! Please show us your work so we can find your error. Do you know what the right hand side reduces to?

so you did the following:

sin 2x + 2 sin^2(45 - x )

= 2 * sin(x) * cos(x) + 2 * $$\displaystyle \left[\frac{\sqrt{2}}{2} * cos(x) - \frac{\sqrt{2}}{2} * sin(x) \right]^2$$

After that I, I would factor out the (√2)/2 and realize that [(√2)/2]2 = 1/2 and continue.....
I have continued and now after factoring out the (√2)/2 and realize that [(√2)/2]2 = 1/2
I have 2 sinx cos x + 2 (1/2 cos^2x - 2sinxcosx + 1/2sin^2x)
reduced to 2sinx cos x + cos^2 x - 2sinxcosx + sin^2x
= sin^2x + cos^2x

is this correct?

Subhotosh Khan

Super Moderator
Staff member
I have continued and now after factoring out the (√2)/2 and realize that [(√2)/2]2 = 1/2
I have 2 sinx cos x + 2 (1/2 cos^2x - 2sinxcosx + 1/2sin^2x)
reduced to 2sinx cos x + cos^2 x - 2sinxcosx + sin^2x
= sin^2x + cos^2x

is this correct?
No! - Look into that step.

Subhotosh Khan

Super Moderator
Staff member
It should 4sinxcosx and not 2
No!!

Are you using pen/paper or just staring at the screen?

Show the complete work done by you - step-by-step.