Question

Prove that a cosA + b cosB + c cosC = 4R sinB sinC * sinA

Answer 1

this is formula in trigonometry

Answer 2

Answer:

Step-by-step explanation:

a cosA+b cosB+c cosC

Sigma 2R sinA cosA

R sigma sin2A

R [sin2A+sin2B+sin2C]

R [2sin(A+B).cos(A–B)+sin2c]

A+B=180–C

R[2sinC Cos(A–B)+2sinC cosC]

R[2sinC[cos(A–B)+cosC]]

R[2sinC(cos(A–B)–cos(A–B)]

R[2sinC(2sinAsinB)]

4RsinAsinBsinC