‌a+b+c=18k Substituting b+c=11k in a+b+c=18k, we get a+11k=18k or a=7k Substituting a=7k in c+a=12k, we get c+7k=12k‌ or ‌c=5k Substituting c=5k in b+c=11k, we get b+5k=11k‌ or ‌b=6k ∴‌‌a=7k,b=6k and c=5k Using cosine rule, we get cos‌A‌=‌