y=b*((x-0)*(x-10)*(x-15))/((5-0)*(5-10)*(5-15))+c*((x-0)*(x-5)*(x-15))/((10-0)*(10-5)*(10-15))+d*((x-0)*(x-5)*(x-10))/((15-0)*(15-5)*(15-10))

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