Question

please solve (b) with steps.

Answer 1

The large triangle is equilateral (as all sides are 10cm long),
The four smaller triangles formed are also equilateral triangles that are congruent to each other (the three corner triangles are congruent due to SAS rule and since this means DE=EF=FD, triangle DEF is also congruent to the corner triangles),
Hence side DE is equal to 5cm.