A:I'll eat three hot dogs.
B:I'll take three hot dogs.
(Ⅰ)若a,b,c成等差数列,证明:sinA+sinC=2sin(A+C);
(Ⅱ)若a,b,c成等比数列,且c=2a,求cosB的值.
要求:文通字顺,内容具体,有真情实感,题目自拟。(不要出现真实的校名、人名)