Question

----cosb=1/2[cos(a+b)+cos(a-b)]
cosa
sinb
sina
cosb

Answer 1

can you expand cos(a+b) ???

Answer 2

or you know the cos C & D formula ???

Answer 3

cos(a+b)=cosacosb-sinasinb

Answer 4

yes and cos(a-b)??

Answer 5

cos(a-b)=cosacosb+sinasinb

Answer 6

neither work because there isnt sin in the inital equation and it isnt adding up for me

Answer 7

now put these values in equation

Answer 8

Im guessing its sina that is the answer just on estimation

Answer 9

replace cos(a+b) by its new value and similar with other ..

Answer 10

just follow you will learn dear...leave the ans it will come

Answer 11

can u replace ??

Answer 12

whats its new value?

Answer 13

cos(a+b)=cosacosb-sinasinb

Answer 14

you know now what u need to use in place of cos(a+b)

Answer 15

got it ??

Answer 16

replace cos(a+b) by cosa cosb-sinasinb

Answer 17

\[\frac{ 1 }{ 2 }[cosa cosb -sina sinb +cosacosb+sinasinb]\]

Answer 18

yeah we just replace the identity by its expansion

Answer 19

so you gott it??

Answer 20

yeah, but now what?

Answer 21

you see that sin terms can cancel each other??

Answer 22

so 1/2[cosacosb+cosacosb]

Answer 23

1/2[cosacosb+cosacosb] can be further simplified to....

Answer 24

idk:( @superdavesuper

Answer 25

cosacosb+cosacosb = 2cosacosb right?

Answer 26

yes, sorry it took me so long to get back. Can you still walk me through this!??:) @superdavesuper

Answer 27

@cutiecomittee123 plz re-read the posts above - its really all there :)

Answer 28

so the answer is cosa