A BOY GOES TO SCHOOL AT X M/S AND COMES TO HOME AT Y M/S. HIS AVERAGE SPEED FOR THE WHOLE TRIP IS (2XY)/X+Y.
BUT I DUN UNDERSTAND. CAN ANYONE EXPLAIN THE ANSWER TO ME?? WHY IS (2XY)/X+Y?? THANKS!^^
ce maths problem pls help!
2009-03-27 10:29 pm
回答 (1)
2009-03-27 11:02 pm
✔ 最佳答案
Speed = distance/times Let the distance of the whole trip be 2D,
then he need times D/X + D/Y in the whole trip
So his average speed = 2D / D/X + D/Y
= 2 / (1/X + 1/Y)
= 2 / [(X+Y)/XY ]
= 2XY / (X+Y)
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