posted by Tareena-- check my answer please .
Two Planes leave an airport at the same time, one flying 300km/h and the other at 420km/h
The angle between their flight paths is 75 degrees after three hours, how fart apart are they?
I don't see your answer to check.
The law of cosines would work well on this problem. You know two sides (900 and 1260 km) and the included angle (75 deg) of a triangle.
so it would be c^2=a^2+b^2-2ab cos C
is it right?
No. It isn't right. See my answer to your later post.