Question

one more vector questions, please help
|a|=2|b|
(a+b) is vertical to (2a-5b)
ask for the angle between a and b
(a and b are not 0)

Answer 1

@ZeHanz can you help again

Answer 2

for angle between a and b you can use cosine formula (do you know it?)
now to fill in the missing quantities in formula use (a+b) . (2a-5b) = 0 (because they are vertical/perpendicular)

try it if you are struck we`ll analyze in detail

Answer 3

*addition
you don`t need cosine formula for this just start with the second hint

Answer 4

ok i'll try, please give me a moment
thank you very much

Answer 5

oh ok
i forgot what way did i use before but i'll think it over with your hint again

Answer 6

use what ?
well just go step by step with second hint and you`ll be trough in minutes

Answer 7

ok,
i know the formula of cos
and i also know they are vertical so they dot and get 0
but what can i do with 2a^2=5b^2
sorry i don't know where I am stuck

Answer 8

ok lets try step by step
check this out

(a+b) . (2a-5b) = 0
2a^2 - 5a.b - 2a.b - 5b^2 = 7a.b
3b^2 = 7ab cos0 ( here a and b on right side is magnitude of vector)
clear till this step?

Answer 9

*\[\cos \theta\] instead of cos 0

Answer 10

sorry, why 2a^2-3ab-5b^2=7ab

Answer 11

oh never mind, thank you very much, i know what had happened.

Answer 12

i got it

Answer 13

good !! (y)

Answer 14

altough 2nd part 2a^2 - 5a.b - 2a.b - 5b^2 = 7a.b should have been 2a^2 - 5a.b - 2a.b - 5b^2 = 0