請教數條數學題?10點!!!!

1)若方程式y=mx+c的直線圖像通過(2,2)和(-1.-7)。
求m和c的值。備註:根據提供的資料,先定出聯立方程。
2 = 2m + c … (1)
-7 = -m + c … (2)
(1) – (2) => 9 = 3m
m = 3
Sub into (1), 2 = 6 + c
c = -4
2) {x + 2y = 10 … (i)
{2x – y = 5 … (ii)
(i) * 2 => 2x + 4y = 20 … (iii)
(iii) – (ii) => 5y = 15
y = 3
Sub into (i) => x + 6 = 10
x = 4
3)小船逆流行駛24km需時3小時。回程時,順流行駛2小時便
到達。小船及水流的速度分別是什麼?
備註: 逆流時船的速率 = 船的速率 – 水流的速率
設小船及水流的速度分別是b km/h 及c km/h
順流時船的速率 = b + c = 24/2 = 12 … (1)
逆流時船的速率 = b – c = 24/3 = 8 … (2)
(1) + (2) => 2b = 20
b = 10
Sub into (1), 10 + c = 12
c = 2
小船及水流的速度分別是10km/h及2km/h
4)某正多邊形的每一個內角都是其外角的三倍。
(a)它的外角是x度,內角是180 – x = 3x
180 = 4x
x = 45
外角是45度
(b)這個正多邊形共有360 / 45 = 8邊
5) 將(5x2 + 6x – 3) – 2(x2 – 4x + 1)簡化是3x2 + 14x – 5
6)利用消元法解聯立方程
{題目中有錯,y = 2x = 5 可能是y + 2x = 5}
{y + 2x = 5 … (1)
{x – 2y = -4 … (2)
(1)*2 => 2y + 4x = 10 … (3)
(2) + (3) => 5x = 6
x = 1.2
Sub into (1)
y + 2.4 = 5
y = 2.6
{亦可能是y – 2x = 5}
{y – 2x = 5 … (1)
{x – 2y = -4 … (2)
(1)*2 => 2y – 4x = 10 … (3)
(2) + (3) => -3x = 6
x = -2
Sub into (1)
y + 4 = 5
y = 1

2009-08-28 00:02:05 補充：
5) 將(5x^2 + 6x – 3) – 2(x^2 – 4x + 1)簡化是3x^2 + 14x – 5