If A = c o s θ - s i n θ s i n θ c o s θ , then the matrix A - 50 when θ = π…

If A=cosθ-sinθsinθcosθ , then the matrix A-50 when θ=π12, is equal to:
  1. 3212-1232
  2. 1232-3212
  3. 32-121232
  4. 12-323212

Solution

A=cosθ-sinθsinθcosθA=cosθ-sinθsinθcosθ=cos2θ+sin2θ=1

And adjA=cosθsinθ-sinθcosθ

A-1=1AadjA=cosθsinθ-sinθcosθ

A-2=A-12=A-1A-1

=cosθsinθ-sinθcosθcosθsinθ-sinθcosθ

=cos2θ-sin2θ2sinθcosθ-2sinθcosθcos2θ-sin2θ

=cos2θsin2θ-sin2θcos2θ

It is visible that

A-1n=cosnθsinnθ-sinnθcosnθ

A-50=A-150=cos50θsin50θ-sin50θcos50θ

Now at θ=π12,sin50θ=sin50π12=sinπ6=12

cos50θ=cosπ6=32

A-50=3212-1232

Asked in: JEE Main 2019 (09 Jan Shift 1)

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