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Theorem related to triangle
Theorem1) The ratio of perimeter of two similar triangle is equal to the ratio of their corresponding side
Theorem 2) the ratio of area of two similar triangles is equal to the ratio of their square of corresponding side
Theorem 3) ratio of area of two similar triangles is equal to the ratio of square of perimeter of triangle.
Let us first prove theorem1)
we know that ratio of corresponding sides of similar triangle are equal
so,
AB/PQ =BC/QR=AC/PR
if we take out the ratio of the ratio of their corresponding sides we get
AB/PQ :BC/QR :AC/PR
=1:1:1
so, from ratio proportion theorem we can say
(AB+BC+AC)/(PQ+QR+PR) =AB/PQ=BC/QR=AC/PR
so,first theorem is proved.....
Let prove the second theorem
Consider the ∆ABC and ∆PQR again
draw AX and PY perpendicular to BC and QR respectively
now in. ∆ABX and in ∆PQY, we can see
angleABX=anglePQY (∆ABC and ∆PQR are similar)
also,angleAXB=anglePYQ (both are eqaul to 90)
by AA similarity we can say
both the traingle are similar
so,
AB/PQ=AX/PY
also,AB/PQ=BC/QR=AC/PR
so,
AX/PY=AB/PQ=BC/QR=AC/PR
ar(∆ABC)=1/2×BC×AX
ar(∆PQR)=1/2×QR×PY
taking the ratio of both the area we can say
ar(∆ABC)/are(∆PQR)=BC/QR ×AX/PY
also we know that
BC/QR=AX/PY
hence
ar(∆ABC)/are(∆PQR)=BC/QR ×BC/QR=(BC/QR)^2
hence second theorem is proved
If we relate the first and second theorem we can say,,,third theorem is exactly right
From,the theorem3 (which is proved above)
we can say
This will be the required ratio of area of triangle
Answer:
Since ∆ABC and ∆PQR are similar
the ratio of there corresponding sides is same
AB/PQ = BC/QR = AC/PR
Let
AB/PQ = BC/QR = AC/PR = K
∴AB = KPQ
BC = KQR
AC = KPR
Now perimeter of ∆ABC = AB + BC + AC
= KPQ + KQR + KPR
= K (PQ+QR+PR)
= K × Perimeter of ∆ PQR
Therefore,
Perimeter of ∆ABC/Perimeter of ∆PQR = K
So we can write,
AB/PQ = BC/QR = AC/PR = K. ...(1)
We know that
Ratio of area of similar triangles is equal to square of its corresponding sides.
Therefore,
Area of ∆ABC/Area of ∆PQR = (AB/PQ)²
Hence by putting the value of AB and PQ is 35cm and 45cm
Area of ∆ABC/Area of ∆PQR = (35/45)²
= (7/9)²
= 49/81
So, the area of ratio of two similar triangles is 49:81.
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