sin15 = SIN（45-30）=√2/ 2 *√3/ 2-1 / 2 *√2/ 2 =（√6-√2）/ 4
cos15 = COS（45-30）=√ 2/2 *√3/ 2 + 1/2 *√2/ 2 =（√6+√2）/ 4
最长边的最短边按照
（比例√6,√ 2）/ 4：=（√6+√2）/ 4:1
=√6-√2：√6+√2：4
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