率 比 及變分 一問

a = kb ............(1)
b = h(a - c) ..........(2) where h, k are constants.
Sub (2) into (1)
a = kh(a - c) = kha - khc
khc = kha - a = a(kh - 1)
so a = khc/(kh - 1)
Since kh/(kh - 1) is also a constant, so a varies directly as c. Ans. is (a).

a,b是正變,b與(a-c)是正變,可看為a與c是正變,

其實同一般的加減乘除看法一樣, 如a增加, b亦

增加, 就是正變的意思, 如a增加, b減少, 就是反變,

如a,1/b是正變, a增加時b會減少, 所以這法則可以

定出a與c是正變的, 因為a,b是正變,b與(a-c)是正變

這兩條件是一般的正變, 相合了也是正變, 因為加減

對正反變是沒影響的, 只有乘除, 分數, 正負有影響