请解答---**Test Information:** 2024学年安阳实验中学七下第4章《因式分解》单元检测 **Question Stem:** 18. 如图1, 有两种边长分别为a, b (a > b) 的正方形. 如图2, 现将1个边长为a的正方形和4个边长为b的正方形无重叠地放在长方形ABCD中. 已知CH=AE, 则AB:BC = 3:2, 记"L"型阴影面积为S1, 四边形NBFM面积为S2, 若S1-S2=20, a和b为正整数, 则"L"型阴影的周长 = _______. **Other Relevant Text:** (Refers to 图1 and 图2, which are not provided in the image.) (Refers to an "L" shaped shaded area and a quadrilateral NBFM within a rectangle ABCD, which are parts of 图2.) Given conditions: CH=AE, AB:BC=3:2, S1-S2=20, a and b are positive integers. Question: Find the perimeter of the "L" shaped shaded area. **Options:** (No options are visible in the provided image.) **Diagram Description:** The image contains text describing a geometry problem involving squares placed in a rectangle and shaded areas. It refers to two figures, 图1 and 图2, which are not present in the image. The text describes the elements within 图2 as a rectangle ABCD containing one square of side length a and four squares of side length b placed without overlap, and mentions specific points (E, H, N, B, F, M) forming shaded regions ("L" shaped and quadrilateral NBFM). Without the figures, a detailed description of the shapes and their relative positions is not possible based solely on the provided image. **Mathematical Formulas/Equations:** S1 - S2 = 20