Question

Without using trigonometric tables, prove that:

cos²75° + cos²15° = 1

Answer 1

LHS = cos²75° + cos²15°

= cos²(`90^circ - 15^circ) + cos^2 15^circ`

 = `sin^2 15^circ + cos^2 15^circ`

= 1 

= RHS