0.5 x base length x vertical height
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The area of an equilateral triangle is A=sqrt(3)*(l^2)/4, l is the length and A is the area multiply both sides by 4/sqrt(3) and get 4*A/sqrt(3)=l^2 take the square root of both sides and get l = sqrt(4*A/sqrt(3))
The altitude of an equilateral triangle bisects the base. So, if the sides of the triangle were l cm, the altitude forms a right angled triangle with sides h, l/2 and hypotenuse l cm. Then, by Pythagoras, h2 = 3l2 / 4 so that h = l*sqrt(3)/2 and then area = l*h/2 = l*[l*sqrt(3)/2]/2 =l2*sqrt(3)/4
180 degrees in any plane triangle.
Three
I hope you want to know the Perimeter. Perimeter is the total length of the boundary of the region bounded by a shape. For a rectangle it is the sum of the 4 bounding sides, or 2*(L+B), where L is Length of the rectangle and B is Breadth of the rectangle. For a Triangle it is the sum of the 3 sides. If you consider an equilateral triangle. By property the 3 sides of an equilateral triangle are equal. Hence the Perimeter of an equilateral triangle is denoted as; 3*a, where a is the length of one of the sides of the triangle. It is possible that the perimeter of a rectangle is same as that of many different types of triangles. We can formulate a relationship for a special case where the perimeter of a rectangle is equal to the perimeter of an equilateral triangle; P(R) = P(ET), P(R) is perimeter of rectangle and P(EQ) is perimeter of Equilateral triangle. P(R)=2(L*B) = P(EQ) = 3*a; hence, a = (2/3)*(L*B) = P(R)/3. i.e., the sides of the Equilateral triangle are one thirds of the perimeter of the rectangle.