Triangles
Category : 10th Class
TRIANGLES
FUNDAMENTALS
Similar figures:
Congruent figures:
Similar Triangles:
e.g., If in \[\Delta \,ABC\]and \[\Delta \,PQR\]
\[\angle A=\angle P,\angle B=\angle Q,\angle C=\angle R\]
and \[\frac{AB}{PQ}=\frac{BC}{QR}=\frac{AC}{PR},\] then, \[\Delta \text{ }ABC\sim \Delta \,PQR;\] where symbol \[\sim \] is read as ‘is similar to’.
(i) \[\angle A=\angle D,\angle B=\angle E\] (called AA similarly)
Or
(ii) \[\frac{AB}{DE}=\frac{BC}{EF}=\frac{AC}{DF}\] (called SSS similarity)
Or
(iii) \[\frac{AB}{PQ}=\frac{BC}{QR}\] and \[\angle B=\angle Q\] (called SAS similarity)
Any one of the above three, would be sufficient for proving similarity.
Conversely: If \[\Delta \text{ }ABC\]is similar to\[\Delta \text{ }PQR\], then
\[\angle A=\angle D;\angle B=\angle E;\angle C=\angle Q\] and \[\frac{AB}{DE}=\frac{BC}{EF}=\frac{AC}{DF}\]
Then, \[\angle A=\angle D;\text{ }\angle B=\angle E;\text{ }\angle C=\angle F\]
and, \[AB=DE,BC=EF\] and \[CA=FD\]
However, to prove congruency, we need to prove any one of the following only:
(i) \[\text{ }AB=DE,\text{ }BC=EF\And CA=FD\](SSS congruency)
or
(ii) \[AB=DE;\text{ }BC=EF\And \angle B=\angle E\] (SAS congruency)
or
(iii) \[\angle A=\angle D;\angle B=\angle E\] and \[AB=DE\](ASA congruency)
How to look for similarity and congruency of angles
While looking for similarity and congruency, you should not only see external appearance but also which of the corresponding angles are equal (or, which of corresponding sides are in the same ratio).
In the above figure, \[\angle B=\angle F;\text{ }\angle C=\angle E\]and \[\angle A\cong \angle D;\] thus \[\Delta \,ACB\cong \Delta \,DEF\](and not\[\Delta \,ABC\cong \Delta \,DEF\])
Mathematical statement of the theorem\[\frac{AD}{DB}=\frac{AE}{EC}\] (where\[DE\parallel ~BC\])
i.e., if in \[\Delta \text{ }ABC\]as shown above, \[DE\parallel BC\Rightarrow \frac{AD}{DB}=\frac{AE}{EC}\]
Converse of Basic Proportionality Theorem:
Mathematical statement of the theorem if in \[\Delta \,ABC\](as shown above),
\[\frac{AD}{DB}=\frac{AE}{EC}\Rightarrow DE\parallel BC)\]
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