Task/26548443
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18.
9cos2α =9(2cos²α -1) =|| cosα =1/3 || =9 (2*(1/3)² -1) =9 (2/9 -1) = 2 -9 = -7.
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19.
- 47cos2α = - 47(2cos²α -1) =|| cosα = - 0,4 || = - 47(2*(-0,4)² -1) =
- 47(0,32 -1) = 47 - 15,04 = 31,96 .
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20.
cosα =- √(1-sin²α) = - √(1-(2√6 /5)² ) = - √(1- 24 /25 ) = 1/5 .
* * * α ∈(π/2 ; π) →cosα< 0 * * *