代数-方程

代数-方程

flyfish 2015-11-19 笔记

方程（equation）：含有未知量的等式。
一次方程（simple equation）：未知量的最高次数是1次的方程。
二次方程（quadratic equation）：未知量的最高次是2次的方程。

一
If 3 more than x is 2 less than 9，what is the value of x？
x+3=9-2

二
A restaurant has 19 tables that can seat a total of 84 people. Some of the tables seat 4 people and the others seat 5 people. How many tables seat 4 people？

假设4人桌有x张，则5人桌有19-x张。4x+5（19-x）=84，x=11

三
The cost of a telephone call using a long-distance carrier A is ＄1.00 for any time up to and including 30 minutes and ＄0.07 per minute thereafter. The cost using long-distance carrier B is ＄0.06 per minute for any amount of time. For a call that lasts t minutes，the cost using carrier A is the same as the cost using carrier B. If t is a positive integer greater than 30，what is the value of t？

A的花费由2部分组成：30分钟以内的基本费用以及超出30分钟的额外费用，
所以A的费用表达式为1+（t－30）·0.07；而B不设基本费用，它的费用表达式为0.06t。
所以1+（t－30）*0.07=0.06*t
t=110