4. Let 0 a,b,c,d π  where b and c are not complementary, such that 
2 cos a + 6 cos b + 7 cos c+ 9 cos d =0 and 2 sin a - 6 sin b + 7 sin c - 9 sin d = 0 then value of  3   cos   a + d cos   b + c

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