Question

____Cosb= 1/2 [ sin(a+b) + sin (a-b)

a. sinb
b. cosa
c. cosb
d. sina

Answer 1

sin(a+b) = sin a cos b + cos a sin b
sin(a-b) = sin a cos b - cos a sin b

Answer 2

add the above 2 equations.

Answer 3

what u get ?

Answer 4

you get... 0 = 0? 0.o wouldn't the minus and plus signs cancil?

Answer 5

how u getting 0 anywhere ?

Answer 6

wait... its a +b ._. sin(a+b) - sin(a-b) =?

Answer 7

sin(a+b) = sin a cos b + cos a sin b
+ sin(a-b) = sin a cos b - cos a sin b
___________________________________________
sin(a+b) +sin (a-b) = 2 sin a cos b + 0

did u get this ?

Answer 8

subsitute sin (a+b) with sin a cos b + cos a sin b
and sin (a-b) with sin a cos b - cos a sin b

Answer 9

wait .why is there a 0 ..i get everything else in that hartnn

Answer 10

cos a sin b - cos a sin b = 0

Answer 11

oooh ..ty ...now continue please?

Answer 12

sin(a+b) +sin (a-b) = 2 sin a cos b
1/2 [sin(a+b) +sin (a-b)] = sin a cos b
and you are done.....

Answer 13

do its d sina right?

Answer 14

yes.

Answer 15

any more doubts ?

Answer 16

NO ..and i actually understood that hartnn !! :D

Answer 17

thats good :)