the answer is "Using the trigonometric identity
sin²x + cos²x = 1 , then sin²x = 1 - cos²x
Consider the left side
sin²a - sin²b
= (1 - cos²a) - (1 - cos²b)
= 1 - cos²a - 1 + cos²b
= cos²b - cos²a
= right side , thus verified"
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