3. Let the radius of the big circle R and the radius of the small circle R be the shadow area, as follows

R^2-r^2 = 40

The area of the ring is π r 2-π r 2 = π (R 2-R 2) = 40 π.

4. Because the areas of the two triangular rows are the same, so

Trapezoidal A 1DBA and trapezoidal DCB 1C 1 have the same area.

As can be seen from the figure, the area of trapezoidal DCB 1C 1 is ((kloc-0/2-4)+12) x 3)/2 = 30.

5.28

Single-layer triangles 16, two-layer triangles 6, three-layer triangles 3, four-layer triangles 1 and five-layer triangles 1, so there are 27 triangles in the drawing.

7。 Let quotient = remainder = x, and x is an integer.

13 x+x & gt； 100 X & gt； 100/ 14 >=8

0 & ltx & lt 13

So x = 8,9, 10, 1 1, 12.

So there are five such numbers.