It is known that in △ABC, AB=AC=12cm, the area of △ABC is 42cm**2, P is any point on BC, the distance from P to AB, AC is x and y, then what is the value of x+y equal to.
已知△ABC中，AB=AC=12cm，△ABC的面积是42cm**2，P是BC上任意一点，P到AB，AC的距离是x和y，那么x+y的值等于多少．
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