[lkppp94]

If the earth was really  round then why a flight from Japan to L.A is not undertaken down the Indian ocean to save time compared to the current trajectory that covers greater distance

[AATW]

Why would it go anywhere near the Indian Ocean? The current trajectory, according to this site, is across the Pacific Ocean.

http://www.travelstart.co.za/blog/worst-flights-for-jetlag/

As you'd expect on a globe, the path is a curve (as represented on a 2D map)

[Tumeni]

Japan is 138 East, LA is 118 W.

The number of degrees to travel, if you go West over the Indian Ocean from Japan, and don't stop until LA, is the sum of these figures - 138+118 = 256 degrees.

Since there are 360 degrees around the Earth, if you go East from Japan, it's (360-256 =) 104 degrees that way.  Less than half the distance if you go over the Pacific.

Some account should be taken of latitude as well, but that's good enough for an approximation

[Kappoapsis]

If the earth was really  round then why a flight from Japan to L.A is not undertaken down the Indian ocean to save time compared to the current trajectory that covers greater distance

maybe look on google earth to figure that one out yourself