SOLUTION: have an equilateral triangle with sides of 13 in. How do I find the area? I know formula is A=1/2bh. Base is 13 but what is height?

Draw it:



Draw the height (in green), which will divide the bottom
13 in. bottom side into two 6.5 in. parts:




If we look at only the left half of the triangle, we
see that it is a right triangle:



The Pythagorean theorem applies to right triangles
and says that

 6.5² + h² = 13²
42.25 + h² = 169  
        h² = 126.75
         h = 
         h = 11.25833025     

Now use the formula:

   A = bh
   A = (6.5)(11.24833025)
   A = 36.58957331 square inches

Edwin