Question

In a ∆DEF, if ∠E = 105° and ∠F = 40°, find ∠D.

Answer 1

Sum of the angles of any triangle is 180°.
In ∆DEF:
$$\begin{gathered} \angle \mathrm{D}+\angle \mathrm{E}+\angle \mathrm{F}=180° \\ \angle \mathrm{D}+105°+40°=180° \\ \mathrm{or}\angle \mathrm{D}=180°-(105°+40°) \\ \mathrm{or}\angle \mathrm{D}=35° \end{gathered}$$