若a:b:c=3:2:1,a2b b2c c2a=200.求：3a 7b - 4c的值（注：a2b等中的2是平方）

热心网友

a:b:c = 3:2:1 === a = 3k,b = 2k, c = k, (k 0)a2b+ b2c+ c2a=200 == 3k*2k+(2k)^2 * k + k^2 * 3k = 200== k = 2== 3a+7b-4c = 19k = 38

热心网友

a:b:c=3:2:1---a=3c;b=2c.a^2*b+b^2*c+c^2*a=200---18c^3+4c^3+3c^3=200---25c^3=200---c^3=8---c=2;a=6;b=4---3a+7b-4c=3*6+7*4-4*2=38