解：?$(1)$?如圖所示
?$(2)$?設(shè)?$OC = x n mile$?
因海監(jiān)船與漁船速度相同，故?$BC = AC$?
?$OA = 90 n mile，$?則?$AC = 90 - x$?
在?$Rt\triangle OBC$?中，?$OB = 30 n mile$?
由勾股定理得?$30^2 + x^2 = (90 - x)^2，$?解得?$x = 40$?
則?$BC = 90 - 40 = 50 n mile$?
答：海監(jiān)船的航程?$BC$?的長(zhǎng)為?$50 n mile。$?
