Ex 3.3, 6 - Prove that cos (pi/4 - x) cos (pi/4 - y) - Chapter 3 - (x + y) formula

  1. Chapter 3 Class 11 Trigonometric Functions
  2. Serial order wise
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Ex 3.3, 6 Prove that: cos ( /4 ) cos ( /4 ) sin ( /4 ) sin ( /4 ) = sin ( + ) Taking L.H.S We know that cos (A + B) = cos A cos B sin A sin B The equation given in Question is of this form Hence A = ( /4 ) B = ( /4 ) Hence cos ( /4 ) cos ( /4 ) sin ( /4 ) sin ( /4 ) = cos [( /4 )" " +( /4 )] = cos [ /4 + /4 ] = cos [ /4+ /4 ] = cos [ /2 ( + ) ] Putting = 180 = cos [(180 )/2 ( + ) ] = cos [90 ( + ) ] = sin ( + ) = R.H.S Hence proved

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