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Prove the following identity:

(a) [tex]\[(a \cos A + b \sin A )^2+(a \sin A - b \cos A )^2= a ^2+b^2\][/tex]

Sagot :

GinnyAnswer
To prove the identity [tex]\((a \cos A + b \sin A )^2 + (a \sin A - b \cos A )^2 = a^2 + b^2\)[/tex], let's follow a step-by-step approach:

1. Expand [tex]\((a \cos A + b \sin A )^2\)[/tex]:

Calculate the square of the first term.
[tex]\[ (a \cos A + b \sin A)^2 = (a \cos A)^2 + 2(a \cos A)(b \sin A) + (b \sin A)^2 \][/tex]
Simplifying each term we get:
[tex]\[ (a \cos A)^2 = a^2 \cos^2 A \][/tex]
[tex]\[ 2(a \cos A)(b \sin A) = 2ab \cos A \sin A \][/tex]
[tex]\[ (b \sin A)^2 = b^2 \sin^2 A \][/tex]
So,
[tex]\[ (a \cos A + b \sin A)^2 = a^2 \cos^2 A + 2ab \cos A \sin A + b^2 \sin^2 A \][/tex]

2. Expand [tex]\((a \sin A - b \cos A )^2\)[/tex]:

Calculate the square of the second term.
[tex]\[ (a \sin A - b \cos A)^2 = (a \sin A)^2 - 2(a \sin A)(b \cos A) + (b \cos A)^2 \][/tex]
Simplifying each term we get:
[tex]\[ (a \sin A)^2 = a^2 \sin^2 A \][/tex]
[tex]\[ -2(a \sin A)(b \cos A) = -2ab \sin A \cos A \][/tex]
[tex]\[ (b \cos A)^2 = b^2 \cos^2 A \][/tex]
So,
[tex]\[ (a \sin A - b \cos A)^2 = a^2 \sin^2 A - 2ab \sin A \cos A + b^2 \cos^2 A \][/tex]

3. Combine the two expanded expressions:

Add the two results obtained from the expansions.
[tex]\[ (a \cos A + b \sin A)^2 + (a \sin A - b \cos A)^2 \][/tex]
Which equals:
[tex]\[ (a^2 \cos^2 A + 2ab \cos A \sin A + b^2 \sin^2 A) + (a^2 \sin^2 A - 2ab \sin A \cos A + b^2 \cos^2 A) \][/tex]

4. Combine like terms:

First, combine [tex]\(a^2 \cos^2 A\)[/tex] and [tex]\(a^2 \sin^2 A\)[/tex]:
[tex]\[ a^2 \cos^2 A + a^2 \sin^2 A = a^2 (\cos^2 A + \sin^2 A) \][/tex]
Since [tex]\(\cos^2 A + \sin^2 A = 1\)[/tex], this simplifies to [tex]\(a^2\)[/tex].

Next, combine [tex]\(b^2 \sin^2 A\)[/tex] and [tex]\(b^2 \cos^2 A\)[/tex]:
[tex]\[ b^2 \sin^2 A + b^2 \cos^2 A = b^2 (\sin^2 A + \cos^2 A) \][/tex]
Again, since [tex]\(\sin^2 A + \cos^2 A = 1\)[/tex], this simplifies to [tex]\(b^2\)[/tex].

Lastly, combine [tex]\(2ab \cos A \sin A\)[/tex] and [tex]\(-2ab \sin A \cos A\)[/tex]:
[tex]\[ 2ab \cos A \sin A - 2ab \sin A \cos A = 0 \][/tex]

5. Combine the final simplified terms:

[tex]\[ a^2 + b^2 + 0 \][/tex]

Therefore,
[tex]\[ (a \cos A + b \sin A)^2 + (a \sin A - b \cos A)^2 = a^2 + b^2 \][/tex]

We have successfully proven the identity.
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