1.

Evaluate \( \begin{bmatrix} cos 65 ^\circ & sin 65^\circ\\[0.3em] sin 25^\circ & cos 25^\circ\\[0.3em] \end{bmatrix}\). Evaluate |(cos 65° sin 65°)(sin25°, cos 25°)|

Answer»

By directly opening this determinant 

cos65° × cos25° -sin25° × sin65° = cos(65°+25°) 

∵ cosAcosB - sinAsinB = cos(A+B) 

= cos90° = 0 

∵ cosAcosB - sinAsinB = cos(A+B)



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