Question 1130256
You can use law of cosines which gives you 
opposite side if you have 2 sides and included angle
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Draw the picture. You have 120 degrees between
the flight paths
1st plane;
{{{ a = 250*2}}}
{{{ a = 500 }}} mi
2nd plane:
{{{ b = 300*2 }}}
{{{ b = 600 }}} mi
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{{{ c = sqrt( a^2 + b^2 - 2*a*b*cos( theta )) }}}
{{{ c = sqrt( 500^2 + 600^2 - 2*500*600*cos( 120 )) }}}
{{{ c = sqrt( 25*10^4 + 36*10^4 - 60*10^4*( -1/2) ) }}}
{{{ c = sqrt( 61*10^4 + 30*10^4 ) }}}
{{{ c = 10^2*sqrt( 91 ) }}}
{{{ c = 100*9.5394}}}
{{{ c = 953.94 }}} mi
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check my math. This looks in the ballpark, since
it's a little less than {{{ 500 + 600 = 1100 }}}