A geometry problem by Mehul Chaturvedi

Geometry Level 4

Find The value of Angle DAE


The answer is 45.

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3 solutions

L e t p , q , r b e i n t e g e r s , s u c h t h a t p 2 + q 2 = r 2 ( . . P y t h a g o r e a n t r i p l e s . . ) . A n d Δ A B C 90 45 45. C B d i v i d e d s u c h t h a t C D : D E : E C : : p : r : q . T h e n E A D = 4 5 o . I n t h i s p r o b l e m C D : D E : E C : : p : r : q : : 3 : 5 : 4. I a m n o t g o o d a t t h e o r y o f n u m b e r s . I s t h e r e a n y p r o o f a l r e a d y ? M y p r o o f i s a s u n d e r . W e s a w E A D = a t a n r s s 2 r P r Q . s 2 r P r Q = s 2 1 2 ( s p ) ( s q ) . = p q + 1 2 ( p + q + r ) ( p + q ) = 1 2 ( p 2 + q 2 + r ( p + q ) ) = 1 2 ( r 2 + r ( p + q ) = r 1 2 ( p + q + r ) = r s . E A D = a t a n ( 1 ) = 45 o . \color{#D61F06}{Let ~p, ~q,~r~~be~integers,~such~ that~p^2+q^2=r^2(..Pythagorean ~triples..).\\ And~\Delta~ABC~~90-45-45.~~~~~~~~CB~divided~such~that~~~CD:DE:EC::p:r:q.\\ Then~~\angle~EAD=45^o}.\\ In~this~problem~ CD:DE:EC::p:r:q::3:5:4.\\ I~am~not~good~at~theory~of~numbers.~Is~there~any~proof~already? My~proof~is~as~under.\\ We ~saw~\angle ~EAD=atan\dfrac{rs}{s^2-r_P*r_Q}.\\ s^2-r_P*r_Q=s^2-\frac 1 2 *(s-p)(s-q).\\ =-p*q+\frac 1 2 *(p+q+r)(p+q)\\ =\frac 1 2 *(p^2+q^2+r*(p+q))\\ =\frac 1 2 *(r^2+r*(p+q)\\ =r*\frac 1 2 *(p+q+r)=rs.\\ \therefore~ \angle ~EAD=atan(1)=45*o.

Ahmad Saad
Dec 11, 2016

Fox To-ong
Jan 13, 2015

by ratio and proportion, A B D = 135 8 A D E = 4 5 A E C = 225 8 \angle ABD = {\dfrac{135}{8}}^\circ \angle ADE = 45^\circ \angle AEC = {\dfrac{225}{8}}^\circ

"by ratio and proportion" can you please explain what is this ratio !! How do you get the angles ? Thanks.

Niranjan Khanderia - 2 years, 11 months ago

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