问题 解答题
(1)计算:(-2)2-5×
1
5
+2sin30°

(2)计算:
a
a-1
÷
a2-a
a2-1
-
1
a-1

(3)解方程组:
x+y=1
2x+y=3
答案

(1)原式=4-1+2×

1
2
=4;

(2)原式=

a
a-1
(a+1)(a-1)
a(a-1)
-
1
a-1
=
a+1
a-1
-
1
a-1
=
a
a-1

(3)

x+y=1  ①
2x+y=3  ②

②-①得x=2,

将x=2代入①,得y=-1,

∴原方程组的解为:

x=2
y=-1

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