Verified answer

Ответ:

17/81

Объяснение:

cos4∝=cos²2∝-sin²2∝

cos2∝=cos²∝-sin²∝

sin2∝=2cos∝*sin∝

cos4∝=cos²2∝-sin²2∝=(cos²∝-sin²∝)²-4cos²∝*sin²∝

sin²∝=(1/3​)²=1/9; cos²∝=1-1/9=8/9;

cos4∝=(cos²∝-sin²∝)²-4cos²∝*sin²∝=(8/9-1/9)²-4*(8/9)*(1/9)=49/81-32/81=17/81