解：?$(1)$?設(shè)底邊長(zhǎng)為?$x\ \mathrm {cm}，$?則腰長(zhǎng)為?$2x\ \mathrm {cm}$?
∴?$x+2x+2x=20，$?解得?$x=4$?
∴底邊長(zhǎng)為?$4\ \mathrm {cm}，$?腰長(zhǎng)為?$8\ \mathrm {cm}$?
∴這個(gè)等腰三角形三邊的長(zhǎng)分別是?$8\ \mathrm {cm}，$??$8\ \mathrm {cm}，$??$4\ \mathrm {cm}$?
?$ (2) (20-5)÷2=7.5(\ \mathrm {cm})$?
能，等腰三角形的三邊分別是?$7.5\ \mathrm {cm}，$??$7.5\ \mathrm {cm}，$??$5\ \mathrm {cm}$?
