三角函數如何應用日常生活中 ??

If you want to know the approximate height of the Bank of China in Central, you can estimate the distance between you and the building (let it be y), then you measure the angle of elevation of the Bank of China (let it be x), so height of the building is y tan x .
Try another example: If you want to know the width of the Sing Mun River at Shatin. You can measure the angle of a spot (say the floating restaurant) on one side of the river from the other side of the river, then walk a distance d, again measure the angle of the same spot. So width of the river = d tan x tan y /(tan x + tan y), where x and y are the 2 angles and d is the distance walked.