cos²α-sin²β/sin²αsin² β=cot² α cot² β -1

cos²α-sin²β/sin²αsin² β=cot² α cot² β -1
cos²α-sin²β/sin²αsin² β=cot² α cot² β -1

cos²α-sin²β/sin²αsin² β=cot² α cot² β -1
左=cot² α cot² β -1=cos²αcos²β/sin²αsin²β-1
=[cos²αcos²β-sin²αsin²β]/sin²αsin²β
=[cos²α(1-sin²β)-sin²β(1-cos²α)]/sin²αsin²β
=cos²α-sin²β/sin²αsin² β=右