# Thread: Hard - Trigonometric function

1. ## Hard - Trigonometric function

Help me solve this question. Can't seem to grasp the formula

4)Given that sin 55degrees = 0.819 and cos20degrees = 0.94,
show how you would use the identities for sin(A+B) and cos(A+B) to find the following:
a) sin 75degrees
b) cos 35 degrees
c) tan 35 degrees
Showing the steps you would follow. NOTE: sin^2theta+cos^2theta=1

2. Originally Posted by GAVREED2
Help me solve this question. Can't seem to grasp the formula

4)Given that sin 55degrees = 0.819 and cos20degrees = 0.94,
show how you would use the identities for sin(A+B) and cos(A+B) to find the following:
a) sin 75degrees
b) cos 35 degrees
c) tan 35 degrees
Showing the steps you would follow. NOTE: sin^2theta+cos^2theta=1
sin(A+B) = sinAcosB+cosAsinB

so sin75=sin(55+20)=sin55.cos20+cos55.sin20

cos(A-B) = cosA.cosB+sinA.sinB

so cos35= cos(55-20)=cos55.cos20+sin55.sin20

3. ## Thanks

Thanks for your help again.....Just wondering how you find c) Tan

Originally Posted by harish21
sin(A+B) = sinAcosB+cosAsinB

so sin75=sin(55+20)=sin55.cos20+cos55.sin20

cos(A-B) = cosA.cosB+sinA.sinB

so cos35= cos(55-20)=cos55.cos20+sin55.sin20

4. Originally Posted by GAVREED2
Thanks for your help again.....Just wondering how you find c) Tan
$\displaystyle tan(A-B)=\frac{sin(A-B)}{cos(A-B)}$

or

$\displaystyle tan(A-B) = \frac{tanA-tanB}{1+tanA.tanB}$

5. Thanks mate! Ur a lifesaver