In the given figure, ΔABC is an equilateral triangle and ΔBDC is an isosceles right triangle, right-angled at D. Then ∠ACD = ?
Since we know all the angles in an equilateral triangle is of 60°.
So, ∠ABC = ∠ACB = ∠CAB = 60° …(i)
Also for an isosceles triangle, the angles opposite to equal sides are equal.
So, ∠DBC = ∠DCB = x (let’s say)
Also sum of all angles in a triangle = 180°.
So, in ΔBDC,
∠DBC + ∠DCB + ∠BDC = 180°
x + x + 90 = 180 {since ∠BDC = 90°}
2x = 90
x = 45°
so ∠DCB = 45 …(ii)
And ∠ACD = ∠ACB + ∠DCB = 60° + 45° = 105° {from (i) and (ii)}