As shown in the figure on the platform with a diameter of 100mm two round steel rods embedded in the two sides of the large workpiece, measure the diameter of the large circular workpiece D, measured

As shown in the figure on the platform with a diameter of 100mm two round steel rods embedded in the two sides of the large workpiece, measure the diameter of the large circular workpiece D, measured outside the two round steel rods Solution: As shown in the figure, point O is the center of the circle of the large circular workpiece, connect OC, OD, CD, BC, AD.

∵ ⊙ O is tangent to ⊙ C, ⊙ O is tangent to ⊙ D, AB is the tangent to ⊙ O,

∴∠DAB = ∠AB = 90 ° ∠ADE = 90 °,

∴AD∥BC,

∵AD = BC,<

∴Quadrilateral ABCD is a parallelogram,

∴Quadrilateral ABCD is a rectangle,

∴CD=AB=400mm,

∵OF⊥CD,

∵OD=OC,

∴CE=200mm,

Set the radius of the large circular workpiece to x, OC=x+ 50, then OE=x-50 (mm),

∵OC2=OE2+EC2,

that is, (x+50)2=(x-50)2+40000,

Solve: x=200,

∴the diameter of the large circular workpiece is 400mm,

Written in scientific notation, it can be written as: 4×102.<

So choose D.