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"The triangle ABC is divided into 2 right triangles I and II. ABC is seen to be equal to 1/2 of the rectangle ABNO." -Foster, 1921

Area of Triangle

"The triangle ABC is divided into 2 right triangles I and II. ABC is seen to be equal to 1/2 of the…

"The child sees triangle ACB = triangle ADB, and that I + II = CA - DB; and so he sees that the area of triangle=1/2 area of rectangle whose base and altitude are the same as those of the triangles." -Foster, 1921

Area of Triangle

"The child sees triangle ACB = triangle ADB, and that I + II = CA - DB; and so he sees that the area…

"Fig. 3 shows triangle I = triangle II, III = IV, and so triangle ABC = 1/2 of rectangle ABDE. The fact is realized that the area of a triangle equals 1/2 the product of the base and altitude." -Foster, 1921

Area of Triangle

"Fig. 3 shows triangle I = triangle II, III = IV, and so triangle ABC = 1/2 of rectangle ABDE. The fact…

Illustration used to show how to construct an equilateral triangle, with a given line as a side.

Construction Of Equilateral Triangle

Illustration used to show how to construct an equilateral triangle, with a given line as a side.

Illustration of equal spheres with equal triangles.

Equal Spheres With Triangles

Illustration of equal spheres with equal triangles.

Illustration of equal spheres with equilateral and equiangular triangles.

Equal Spheres With Triangles

Illustration of equal spheres with equilateral and equiangular triangles.

Illustration used to show that two triangles are equal if the three sides of one are equal respectively to the three sides of the other.

Equal Triangles

Illustration used to show that two triangles are equal if the three sides of one are equal respectively…