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if √2cosA=cosB+cos3A & √2sinA=sinB-sin3B,show that sin(A-B)=±1/3

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1
Manmay kumar Mohanty ·

Multiplying the given relations by sinB and cosB and subtracting we get
-√2(sinA cosB - cosA sinB) = sinBcosB (cos2B + sin2 B)
so
sin(A-B) = - (12√2)sin2B ............................(1)
again squaring and adding the given relations, we get
2 x 1 = 1 + (cos6B + sin6B) + 2 ( cos4 B - sin4 B )
1 = 1.(cos4 B + sin4 B - cos2Bsin2B) + 2.1.(cos2B - sin2B)
1 = (1 - 3cos2Bsin2B) + 2cos2B
34(2sinBcosB)2 = 2cos2B
3 sin22B = 8cos2B
3 cos22B + 8cos2B - 3 = 0
(cos2B+3)(3cos2B-1) = 0
cos 2B = 1/3 as cos 2B≠3
sin22B = 1 -cos22B = 1 - 1/9 = 8/9
sin 2B = ± 2√23

hence from (1)
sin ( A - B ) = - (12√2)sin2B = ±13

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