20 10 Hubei college entrance examination mathematics third question 3. In △ABC, a= 15, B = b= 10, ∠A= 60 = 60, why choose d instead of c?
Looking for cosB=? The answer is that C must believe in himself first!
Do CD⊥AB at point D after point C.
In the right triangle ACD
∠A=60
Sina =CD/AC
CD=AC*sinA= 10*√3/2=5√3
In the right triangle BCD
bd^2=bc^2-cd^2=225-75= 150
BD=√ 150=5√6
cosB=BD/BC=5√6/ 15=√6/3