definite integral calculation in ruby

'''
calculate definite integral for given intervals and slices
+startPoint+:: float
+endPoint+:: float
+numberofSlices+:: integer
'''
def integral(startPoint,endPoint,numberofSlices)
        # change for each iteration
        deltaX = (endPoint-startPoint)/numberofSlices
        # total area and iteration area, must be defined here before used
        area, sliceArea = 0.0
        # simple loop
        numberofSlices.times do |i|
                x = startPoint + i * deltaX.to_f
                #the function
                y = 3*x**2
                # area of each slice
                sliceArea = y * deltaX.to_f
                area = area + sliceArea
                i += 1
        end
        return area.round
end
 
puts integral(1.0,3.0,10000)