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Let 0lt=a , b , c ,dlt=pi, where b and ...

Let `0lt=a , b , c ,dlt=pi,` where b and c are not complementary, such that `2cosa+6cosb+7cosc+9cosd=0` and `2sina-6sinb+7sinc-9sind=0,` then the value of `3(cos(a+d))/("cos"(b+c))` is_____

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