Question

cos 70◦ + sin 80◦ = ?

Answer 1

change that cosine into a sine,
\[\cos(90°-u)=\sin (u)\]

Answer 2

then use the \[\sin u+\sin v=2\sin\Big(\frac{u+v}2\Big)\cos\Big(\frac{u-v}2\Big)\]

Answer 3

the choices are
a.\[\sqrt{3} \sin 50◦\]
b.\[\sqrt{3} \cos 50◦\]
c.\[\sqrt{3} \sin 40◦\]
d.\[\sqrt{3} \cos 10◦\]
e.\[\sqrt{3} \sin 10◦\]

Answer 4

can you help me, what is the correct answer?

Answer 5

what is the correct answer?

Answer 6

or maybe it would have been easier to change the sine to a cosine

\[\sin(90°-u)=\cos (u)\]

\[\sin(80°)=\sin (90°-10°)=\cos(10°)\]

Answer 7

\[\cos 70° + \sin 80°\\
=\cos 70° + \cos10° \]
then use

\[\cos(u)+\cos(v)=2\cos\big(\frac{u+v}2\big)\cos\big(\frac{u-v}2\big)\]