Question

Provethat2 cos (A +45°) cos (B-45°) = cos 2A​

Answer 1

Answer:

Step-by-step explanation:

2 cos(A+45)cos (B-45)

=2[cos A cos 45-sin A sin 45][cos B cos 45+sin B sin 45]

=2[cos A ×1/√2-sin A ×1/√2][cos B×1/√2+sin B×1/√2]

=2[1/√2(cos A-sin A)][1/√2(cos B-sin B]

=2×1/√2×1/√2 (cos A-sin A)(cos B+sin B)

=cosA cos B+cos A sin B-sin A cos B-sin A sin B

=cos A cos B-sin A sin B-(sin A cos B-cos A sin B)

=cos (A+B)-sin (A-B)

i think there should be A in place of B.

then

=cos (A+A)-sin (A-A)

=cos 2A-sin 0

=cos 2A

as sin 0=0