Q.
Let X be the solution set of the equation Ax = I, where A=01−14−343−34 and I is the corresponding unit matrix and x⊆N, then the minimum value of ∑cosxθ+sinxθ, θ∈(0,π/2), is ___________.
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answer is 2.
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Detailed Solution
A=01−14−343−34⇒ A2=A⋅A=01−14−343−34⋅01−14−343−34=0+4−30−3+30+4−40−12+124+9−12−4−12+160−12+123+9−12−3−12+16=100010001⇒ A2=I⇒A4=A6=A8=INow Ax = I⇒ x=2, 4, 6, 8, …∴ Σcosxθ+sinxθ =cos2θ+sin2θ+cos4θ+sin4θ+cos6θ+sin6θ+... =cos2θ+cos4θ+cos6θ+....+sin4θ+sin4θ+sin6θ.... =cos2θ1−cos2θ+sin2θ1−sin2θ =cot2θ+tan2θwhich has minimum value 2
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