Question

In: Math

If (sin4x)/2 + (cos4x)/3 = ⅕, then

If (sin4x)/2 + (cos4x)/3 = ⅕, then

(a) tan2x = ⅔

(b) (sin8x)/8 + (cos8x)/27 = 1/125

(c) tan2x = ⅓

(d) (sin8x)/8 + (cos8x)/27 = 2/125

Solutions

Expert Solution

Given (sin⁴x)/2 + (cos⁴x)/3 = ⅕

=> (sin⁴x)/2 + (1 – sin²x)2/3 = ⅕

=> (sin⁴x)/2 + (1 – 2 sin²x + sin⁴x)/3 = 1/5

Put t = sin²x

=> t²/2 + (1-2t+t²)/3 = ⅕

=> 3t² + 2 – 4t + 2t² = 6/5

=> 5t² – 4t + 2 = 6/5

=> 25t² – 20t + 10 = 0

=> (5t-2)² = 0

=> t = 2/5

sin²x = 2/5

cos²x = 1-sin²x

= 1 – 2/5

= ⅗

tan²x  = 2/3

(sin⁸x)/8 + (cos⁸x)/27 = (2/5)4/8 + (3/5)4/27

= (2/54) + (3/54)

= 5/54

= 1/125

Hence option a and b are correct.

 

 


tan²x = ⅔ and (sin⁸x)/8 + (cos⁸x)/27 = 1/125 are correct.

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