# A level OCR maths question help

okay so the question gave me 2 equations of a curve and said find the x coordinate so I know you have to equate them but one of the equations is 15-2^(x+1) and in the mark scheme expand it to be 15-2 x 2^x but wouldn't it be expanded to 15 x -2 x 2^x
(edited 4 months ago)
Original post by esha06
okay so the question gave me 2 equations of a curve and said find the x coordinate so I know you have to equate them but one of the equations is 15-2^(x+1) and in the mark scheme expand it to be 15-2 x 2^x but wouldn't it be expanded to 15 x -2 x 2^x

the other equation is 3 x 2^x for context
(edited 4 months ago)
No
15 - 2*2^x = 15 - 2^(x+1)
so usual precedence/interpretation.

Simple case ...
2^(x+1) = 2*2*.... *2
x+1 times (assuming x is an integer) and subtract it from 15
(edited 4 months ago)
Original post by mqb2766
No
15 - 2*2^x = 15 - 2^(x+1)

Simple case ...
2^(x+1) = 2*2*.... 2
x+1 times (assuming x is an integer)

ohhhh yeahhh whoops
Original post by mqb2766
No
15 - 2*2^x = 15 - 2^(x+1)
so usual precedence/interpretation.

Simple case ...
2^(x+1) = 2*2*.... *2
x+1 times (assuming x is an integer) and subtract it from 15

okay i got up to here but I somehow got an extra 2^x
Original post by esha06
okay i got up to here but I somehow got an extra 2^x

Penultimate line, you only have 15 on the right hand side. Youre still thinking about the 15 multiplying the 2^x which it doesnt.

Its just putting the x's on the left and constant(s) on the right, then solving, as usual.
(edited 4 months ago)
Original post by mqb2766
Penultimate line, you only have 15 on the right hand side.

Its just putting the x's on the left and constant(s) on the right, then solving.

how do i only have 15, where did the 2^x go?
because the line before that all I did was add 2 to both sides so that shouldn't affect the 2^x rightttttt???
Original post by esha06
how do i only have 15, where did the 2^x go?
because the line before that all I did was add 2 to both sides so that shouldn't affect the 2^x rightttttt???

The right hand side is
15 - 2^(x+1)
add 2^(x+1) to both sides gives
3*2^x + 2^(x+1) = 15
and do the "usual" simplification to the left hand side and solve.

If the equation was
3x = 15 - 2x
what would you do? This is little different.
Original post by esha06
okay i got up to here but I somehow got an extra 2^x

You are almost right...
There is something wrong when you try to simplify in line3.
Think what happens when you have 3 lots of 2^x and then try to add up 2 lots of 2^x