# Thread: Trigonometric indentity problem

1. ## Trigonometric indentity problem

Can someone solve this for me i would be very grateful????????

Using the identities for sin(A+B) and cos^2(theta)+sin^2(theta)=1.

Find sine 65 degrees, given that sin 30=0.643 and sin 25=0.423?

2. $\sin(65) = \sin(90-25)= \dots$

3. Pickslides thankyou for your reply however i don't follow, i'm not very good at trig........

4. $\sin(65) = \sin(90-25)= \sin(90)\cos(25)-\sin(25)\cos(90)$

What's next?

5. Originally Posted by GAVREED2
Find sine 65 degrees, given that sin 30=0.643 and sin 25=0.423?
You really need to be careful with what you type. First of all, you wrote "sin 30" and "sin 25", but are they in degrees or radians? I assume that they are in degrees, but you need the degree symbol.

Second, sin 30° is NOT 0.643. It is 0.5. A quick check on the calculator shows that you probably meant sin 40° ≈ 0.643.

And third, you already posted this problem before (http://www.mathhelpforum.com/math-he...em-153105.html), but you neglected to give use sin 40° and sin 25°. Please make sure to copy the ENTIRE problem accurately, because it is frustrating to try to work on a problem with incomplete or incorrect information.

### trigonometric indentity

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