Given that a and B are acute angles, and sin (a + b) = 4 / 5, cos (a-b) = 3 / 5, is the first sin2a of sin2a and cos2a equal to 24 / 25?

I think you've got the wrong number. It should be sin (a-b) = 4 / 5, cos (a + b) = 3 / 5
In this way, we can get - π / 2 from a and B being acute angles

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