Question

what is the value of sin18​

Answer 1

Answer:

Therefore, sin 18° = sin A = −1±5√4

Step-by-step explanation:

Answer 2

LET A= 18°

Therefore, 5A=90°

2A+3A=90°

2A=90°-3A

taking sin on both sides

sin2A=sin(90°-3A)=cos3A

2sinAcosA=4cos^3A-3cosA

2sinAcosA-4cos^3A+3cosA=0

cosA(2sinA-4cos^2A+3) =0

dividing both side by cosA=cos18°which is not equal to 0,we get

2sinA-4(1-sin^2A)+3=0

4sin^2A+2sinA-1=0

it is a quadratic equation

therefore, sinA=-2+/-√2^2-4(4)(1)/2(4)

sinA=-2+/-√4+16/8

sinA=-2+/-2√5/8

sinA=-1+/-√5/4

here, A=18°

so, sin 18°=-1+/-√5/4 or 0.309...

NOTE:+/- MEANS BOTH PLUS AND MINIUS