Who can help me solve these four big problems!

The first ... Congruent triangles, which make use of "the corners and edges are equal, and the two triangles are congruent". The second is to use "the corners and sides are equal, and the two triangles are congruent" and the third is to use "the three sides of the two triangles correspond to the equal, so the two tables are congruent, so the intersection ADC and the intersection ADB are equal, and because the addition of the two angles is equal to 180 degrees, so the AD is perpendicular to BC".

The fourth. (1) It belongs to the body size area, which is equal to 1 2 (a + b) (a + b). (2) Using the trigonometric relationship between tangent and cotangent, we can get the fact that a=b, so these three triangles are all isosceles right triangles...

The first way, the two triangles are congruent (SAS), and the second way, the congruence (SH).

limf(0-)=1=limf（0+）

So existence is 1

The two are not equal and do not exist.

Limits exist or not:

When the limit is infinity, the limit does not exist.

The left limit is not equal to the right limit.

1.If the width is x, the length is x 180

x+180）×2+2x=1560

x=300 then long x+180=480

Length * width = area.

300*480=144000 (square meters).

1 hectare = 10,000 square meters.

A: His area is hectares.

2.Let one inner angle be x and the other inner angle y then the third internal angle is (x-y) and the system of equations (x-y) = 180-x-y

x+y+(x-y)=180

Solve the system of equations to get x=90 y=60 x-y=30Answer: This triangle must be a right triangle.

3.Solution: kilometers).

km) A: Walked in the mountains for hours.

4.Setup: This book has x pages.

45*(1+20%)=54 (pages).

1/4x=54

x=216A: This book has 216 pages.

It turns out that there are x books on the finch cavity of the bookshelf A and Y books on the second shelf.

x+5=5(y-5)

x-5=3(y+5)

x=95y=25

2. When setting uphill, there are x kilometers of uphill roads, and there are y kilometers of noisy downhill roads.

x/20+y/35=9

x/35+20/y=15/2

x=140y=70

The two liters of town are 210 miles apart and 140 uphill when they go

x+7y+z=315 (1)

4x+10y+z=420 (2)

2)-(1) gives x+3y=105 (3).

1)-2*(3).

x+y+z=105

A total of 105 yuan is required.

Do the first question yourself, and see if it doesn't come out.

The second question: 1 (2x3) can be decomposed into (1 2-1 3), the latter can also be decomposed, the middle cancels each other, and finally the first 2 numbers

The third question is similar to the second, but multiplied by 1 2...

Do you understand?