Is sin^6(a)+cos^6(a)=1-3sin^2(a)*cos^2(a) ?

The answer to whether sin^6(a)+cos^6(a)=1-3sin^2(a)*cos^2(a) is True

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prove sin^6(a)+cos^6(a)=1-3sin^2(a)*cos^2(a)

Solution

prove

sin^{6} (a) + cos^{6} (a) = 1 - 3 sin^{2} (a) \cdot cos^{2} (a)

Solution

True

Solution steps

sin^{6} (a) + cos^{6} (a) = 1 - 3 sin^{2} (a) cos^{2} (a)

Manipulating left side

sin^{6} (a) + cos^{6} (a)

Factor

cos^{6} (a) + sin^{6} (a) : (cos^{2} (a) + sin^{2} (a)) (cos^{4} (a) - sin^{2} (a) cos^{2} (a) + sin^{4} (a))

cos^{6} (a) + sin^{6} (a)

Rewrite

cos^{6} (a) + sin^{6} (a)

(cos^{2} (a))^{3} + (sin^{2} (a))^{3}

cos^{6} (a) + sin^{6} (a)

Apply exponent rule:

a^{b c} = (a^{b})^{c}

sin^{6} (a) = (sin^{2} (a))^{3}

= cos^{6} (a) + (sin^{2} (a))^{3}

Apply exponent rule:

a^{b c} = (a^{b})^{c}

cos^{6} (a) = (cos^{2} (a))^{3}

= (cos^{2} (a))^{3} + (sin^{2} (a))^{3}

= (cos^{2} (a))^{3} + (sin^{2} (a))^{3}

Apply Sum of Cubes Formula:

x^{3} + y^{3} = (x + y) (x^{2} - x y + y^{2})

(cos^{2} (a))^{3} + (sin^{2} (a))^{3} = (cos^{2} (a) + sin^{2} (a)) (cos^{4} (a) - sin^{2} (a) cos^{2} (a) + sin^{4} (a))

= (sin^{2} (a) + cos^{2} (a)) (sin^{4} (a) - sin^{2} (a) cos^{2} (a) + cos^{4} (a))

= (cos^{2} (a) + sin^{2} (a)) (cos^{4} (a) + sin^{4} (a) - cos^{2} (a) sin^{2} (a))

Rewrite using trig identities

(cos^{2} (a) + sin^{2} (a)) (cos^{4} (a) + sin^{4} (a) - cos^{2} (a) sin^{2} (a))

Use the Pythagorean identity:

cos^{2} (x) + sin^{2} (x) = 1

= 1 \cdot (cos^{4} (a) + sin^{4} (a) - cos^{2} (a) sin^{2} (a))

Simplify

= cos^{4} (a) + sin^{4} (a) - cos^{2} (a) sin^{2} (a)

Use the Double Angle identity:

cos^{2} (x) - sin^{2} (x) = cos (2 x)

- sin^{2} (x) = cos (2 x) - cos^{2} (x)

= cos^{4} (a) + sin^{4} (a) + cos^{2} (a) (cos (2 a) - cos^{2} (a))

Simplify

cos^{4} (a) + sin^{4} (a) + cos^{2} (a) (cos (2 a) - cos^{2} (a)) : sin^{4} (a) + cos^{2} (a) cos (2 a)

cos^{4} (a) + sin^{4} (a) + cos^{2} (a) (cos (2 a) - cos^{2} (a))

Expand

cos^{2} (a) (cos (2 a) - cos^{2} (a)) : cos^{2} (a) cos (2 a) - cos^{4} (a)

cos^{2} (a) (cos (2 a) - cos^{2} (a))

Apply the distributive law:

a (b - c) = ab - a c

a = cos^{2} (a), b = cos (2 a), c = cos^{2} (a)

= cos^{2} (a) cos (2 a) - cos^{2} (a) cos^{2} (a)

cos^{2} (a) cos^{2} (a) = cos^{4} (a)

cos^{2} (a) cos^{2} (a)

Apply exponent rule:

a^{b} \cdot a^{c} = a^{b + c}

cos^{2} (a) cos^{2} (a) = cos^{2 + 2} (a)

= cos^{2 + 2} (a)

Add the numbers:

2 + 2 = 4

= cos^{4} (a)

= cos^{2} (a) cos (2 a) - cos^{4} (a)

= cos^{4} (a) + sin^{4} (a) + cos^{2} (a) cos (2 a) - cos^{4} (a)

Add similar elements:

cos^{4} (a) - cos^{4} (a) = 0

= sin^{4} (a) + cos^{2} (a) cos (2 a)

= sin^{4} (a) + cos^{2} (a) cos (2 a)

Use the Double Angle identity:

cos (2 x) = 1 - 2 sin^{2} (x)

= sin^{4} (a) + cos^{2} (a) (1 - 2 sin^{2} (a))

Use the Pythagorean identity:

cos^{2} (x) + sin^{2} (x) = 1

cos^{2} (x) = 1 - sin^{2} (x)

= sin^{4} (a) + (1 - sin^{2} (a)) (1 - 2 sin^{2} (a))

Simplify

sin^{4} (a) + (1 - sin^{2} (a)) (1 - 2 sin^{2} (a)) : 3 sin^{4} (a) - 3 sin^{2} (a) + 1

sin^{4} (a) + (1 - sin^{2} (a)) (1 - 2 sin^{2} (a))

Expand

(1 - sin^{2} (a)) (1 - 2 sin^{2} (a)) : 1 - 3 sin^{2} (a) + 2 sin^{4} (a)

(1 - sin^{2} (a)) (1 - 2 sin^{2} (a))

Apply FOIL method:

(a + b) (c + d) = a c + a d + b c + b d

a = 1, b = - sin^{2} (a), c = 1, d = - 2 sin^{2} (a)

= 1 \cdot 1 + 1 \cdot (- 2 sin^{2} (a)) + (- sin^{2} (a)) \cdot 1 + (- sin^{2} (a)) (- 2 sin^{2} (a))

Apply minus-plus rules

+ (- a) = - a, (- a) (- b) = ab

= 1 \cdot 1 - 1 \cdot 2 sin^{2} (a) - 1 \cdot sin^{2} (a) + 2 sin^{2} (a) sin^{2} (a)

Simplify

1 \cdot 1 - 1 \cdot 2 sin^{2} (a) - 1 \cdot sin^{2} (a) + 2 sin^{2} (a) sin^{2} (a) : 1 - 3 sin^{2} (a) + 2 sin^{4} (a)

1 \cdot 1 - 1 \cdot 2 sin^{2} (a) - 1 \cdot sin^{2} (a) + 2 sin^{2} (a) sin^{2} (a)

1 \cdot 1 = 1

1 \cdot 1

Multiply the numbers:

1 \cdot 1 = 1

= 1

1 \cdot 2 sin^{2} (a) = 2 sin^{2} (a)

1 \cdot 2 sin^{2} (a)

Multiply the numbers:

1 \cdot 2 = 2

= 2 sin^{2} (a)

1 \cdot sin^{2} (a) = sin^{2} (a)

1 \cdot sin^{2} (a)

Multiply:

1 \cdot sin^{2} (a) = sin^{2} (a)

= sin^{2} (a)

2 sin^{2} (a) sin^{2} (a) = 2 sin^{4} (a)

2 sin^{2} (a) sin^{2} (a)

Apply exponent rule:

a^{b} \cdot a^{c} = a^{b + c}

sin^{2} (a) sin^{2} (a) = sin^{2 + 2} (a)

= 2 sin^{2 + 2} (a)

Add the numbers:

2 + 2 = 4

= 2 sin^{4} (a)

= 1 - 2 sin^{2} (a) - sin^{2} (a) + 2 sin^{4} (a)

Add similar elements:

- 2 sin^{2} (a) - sin^{2} (a) = - 3 sin^{2} (a)

= 1 - 3 sin^{2} (a) + 2 sin^{4} (a)

= 1 - 3 sin^{2} (a) + 2 sin^{4} (a)

= sin^{4} (a) + 1 - 3 sin^{2} (a) + 2 sin^{4} (a)

Simplify

sin^{4} (a) + 1 - 3 sin^{2} (a) + 2 sin^{4} (a) : 3 sin^{4} (a) - 3 sin^{2} (a) + 1

sin^{4} (a) + 1 - 3 sin^{2} (a) + 2 sin^{4} (a)

Group like terms

= sin^{4} (a) - 3 sin^{2} (a) + 2 sin^{4} (a) + 1

Add similar elements:

sin^{4} (a) + 2 sin^{4} (a) = 3 sin^{4} (a)

= 3 sin^{4} (a) - 3 sin^{2} (a) + 1

= 3 sin^{4} (a) - 3 sin^{2} (a) + 1

= 3 sin^{4} (a) - 3 sin^{2} (a) + 1

= 3 sin^{4} (a) - 3 sin^{2} (a) + 1

Factor

- 3 sin^{2} (a) + 3 sin^{4} (a) : 3 sin^{2} (a) (sin (a) + 1) (sin (a) - 1)

- 3 sin^{2} (a) + 3 sin^{4} (a)

Factor out common term

3 sin^{2} (a) : 3 sin^{2} (a) (sin^{2} (a) - 1)

3 sin^{4} (a) - 3 sin^{2} (a)

Apply exponent rule:

a^{b + c} = a^{b} a^{c}

sin^{4} (a) = sin^{2} (a) sin^{2} (a)

= 3 sin^{2} (a) sin^{2} (a) - 3 sin^{2} (a)

Factor out common term

3 sin^{2} (a)

= 3 sin^{2} (a) (sin^{2} (a) - 1)

= 3 sin^{2} (a) (sin^{2} (a) - 1)

Factor

sin^{2} (a) - 1 : (sin (a) + 1) (sin (a) - 1)

sin^{2} (a) - 1

Rewrite

1

1^{2}

= sin^{2} (a) - 1^{2}

Apply Difference of Two Squares Formula:

x^{2} - y^{2} = (x + y) (x - y)

sin^{2} (a) - 1^{2} = (sin (a) + 1) (sin (a) - 1)

= (sin (a) + 1) (sin (a) - 1)

= 3 sin^{2} (a) (sin (a) + 1) (sin (a) - 1)

= 1 + (- 1 + sin (a)) (1 + sin (a)) \cdot 3 sin^{2} (a)

Rewrite using trig identities

1 + (- 1 + sin (a)) (1 + sin (a)) \cdot 3 sin^{2} (a)

Expand

(sin (a) + 1) (sin (a) - 1) : sin^{2} (a) - 1

(sin (a) + 1) (sin (a) - 1)

Apply Difference of Two Squares Formula:

(a + b) (a - b) = a^{2} - b^{2}

a = sin (a), b = 1

= sin^{2} (a) - 1^{2}

Apply rule

1^{a} = 1

1^{2} = 1

= sin^{2} (a) - 1

= 1 + 3 sin^{2} (a) (sin^{2} (a) - 1)

Use the Pythagorean identity:

1 = cos^{2} (a) + sin^{2} (a)

1 - sin^{2} (a) = cos^{2} (a)

= 1 + 3 sin^{2} (a) (- cos^{2} (a))

Simplify

= 1 - 3 sin^{2} (a) cos^{2} (a)

= 1 - 3 sin^{2} (a) cos^{2} (a)

We showed that the two sides could take the same form

\Rightarrow True

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Frequently Asked Questions (FAQ)

Is sin^6(a)+cos^6(a)=1-3sin^2(a)*cos^2(a) ?
The answer to whether sin^6(a)+cos^6(a)=1-3sin^2(a)*cos^2(a) is True