tolong kerjakan yang no 15 - 18

15 )f(x) = ax + b
f(1) = a + b = 7
f(2) = 2a + b = 9
----------------------(-)
- a = - 2
a = 2

a + b = 7
2 + b = 7
b = 7 - 2
b = 5

f(x) = 2x + 5

f(3) = a
2(3) + 5 = a
6 + 5 = a
a = 11

f(4) = b
2(4) + 5 = b
8 + 5 = b
b = 13

f(6) = c
2(6) + 5 = c
12 + 5 = c
c = 17

nilai 2a - ( b - c )
= 2(11) - ( 13 - 17 )
= 22 - ( - 4)
= 26 ( B )

16) f(x) = 4x + 5
g(x) = ax + 1
f(2) = g(3)
4(2) + 5 = a(3) + 1
8 + 5 = 3a + 1
13 = 3a + 1
13 - 1 = 3a
12 = 3a
a = 4

g(x) = 4x + 1
g(5) = 4(5) + 1 = 20 + 1 = 21 ( B )

17) ax - 4y - 5 = 0
- 4y = - ax + 5
m = - a / - 4
m = a/4
- 3/2 = a/4
a = ( -3 × 4 ) / 2
a = - 12 / 2
a = - 6
nilai 2a + 3
= 2(-6) + 3
= - 12 + 3
= - 9 (A )

18) 3x - 2y + 1 = 0
- 2y = - 3x - 1
m = -3/-2
m = 3/2
syarat gradien tegak lurus
m1 . m2 = - 1
3/2 . m2 = - 1
m2 = - ⅔
pers garis
y - y1 = m ( x - X1 )
y - 2 = - ⅔ ( x + 4 )
y = - ⅔ x - 8/3 + 2 ( dikali 3)
3y = - 2x - 8 + 6
3y + 2x + 2 = 0 ( D )